Passing FDs/handles between processes on Unix and Windows -- a comparison

Handles on Windows are analogous to file descriptors (FDs) on Unix, and both can be passed between processes. However, the way in which handles/FDs can be passed between processes is quite different on Unix and Windows.

In this blog post I'll explain the difference. You might find this useful if you are familiar with systems programming on either Unix or Windows but not both.

Similarities

I'll first explain what's the same on Unix and Windows. Both OSes have a distinction between FD/handle numbers and FD/handle objects.

On both Windows and Unix, each process has its own FD/handle table which maps from FD/handle numbers to FD/handle objects:

• FD numbers are indexes into the FD table. On Unix, an FD number is an int.

  Windows uses the HANDLE type for handle numbers. Though HANDLE is typedef'd to void *, a HANDLE is really just a 32-bit index. (Windows does not use a HANDLE as a pointer into the process's address space.)

• FD objects are what FD numbers map to. User code never gets to see FD objects directly: it can only manipulate them via FD numbers. Multiple FD numbers can map to the same FD object. FD objects are (mostly) reference counted.

  All of this applies to handles on Windows too.

Note: Some people use the alternative terminology that a "file description" refers to the underlying object while "file descriptor" refers to the number. I prefer to add "number" or "object" as a suffix as the way to disambiguate -- it is more explicit, and the term "file description" is not often used.

Differences

The key difference between Unix and Windows is this:

On Unix, FD objects can be sent via sockets in messages. On Windows, handle objects cannot be sent in messages; only handle numbers can.

Windows fills this gap by allowing one process to read or modify another process's handle table synchronously using the DuplicateHandle() API. Using this API involves one process dealing with another process's handle numbers.

In contrast, Unix has no equivalent to DuplicateHandle(). A Unix process's FD table is private to the process. Consequently, on Unix it is much rarer for a process to have dealings with another process's FD numbers.

On Windows, to send a handle to another process, the sender will generally call two system calls:

• Firstly, the sender must call DuplicateHandle() to copy the handle to the destination process. This requires the sender to have a process handle for the destination process. DuplicateHandle() will return a handle number indexing into the destination process's handle table.

• Secondly, the sender must communicate the handle number to the destination process, e.g. by sending a message containing the number via a pipe using WriteFile().

On Unix, to send a handle to another process, the sender will call just one system call, sendmsg(), for sending a message via a Unix domain socket:

• The sender's call to sendmsg() specifies an FD number to send via the cmsg/SCM_RIGHTS interface. The kernel translates this to an FD object, and copies the reference to this object into the socket's in-kernel message buffer.

• The receiver can call recvmsg() to receive the message. recvmsg() will remove the FD object from the socket's message buffer and add the FD object to the calling process's FD table, allocating a new FD number within that table.

Implications

• Cycles and GC: On Unix, it is possible to create a reference cycle from a socket to itself, via the socket's own message buffer. This means it is possible that a socket FD object is only referenced by its own message buffer. Reference counting alone wouldn't be enough to free an unreachable socket such as that. Linux therefore has an in-kernel garbage collector (GC) that handles freeing unused socket FD objects when there are reference cycles.

  In contrast, Windows doesn't need a GC for handling cycles between handles.

• Process handles: Windows has a notion of process handles, which are used by the DuplicateHandle() API. Unix does not have an equivalent concept of process FDs as standard, although FreeBSD has process FDs (added as part of the Capsicum API).

• Sandboxing: It is easier to handle sandboxed processes with the Unix model than with the Windows model.

  On Unix, it is easy for mutually distrusting processes to exchange FDs, because the kernel handles translating FD numbers between processes.

  On Windows, if two processes want to exchange handles, one will generally have a process handle for the other process, which gives the first process control over the latter process. An alternative is to use a broker process for exchanging handles. Chrome's Windows sandbox implements this for exchanging handles between sandboxed processes (see BrokerDuplicateHandle() in content/common/sandbox_win.cc).

• Namespace hazard: Windows' handle-passing model creates a potential hazard: If DuplicateHandle() gives you a HANDLE that's an index into another process's handle table, you must be careful not to treat it as an index into your process's handle table. For example, don't call CloseHandle() on it, otherwise you might close an unrelated handle.

  It is much rarer for this hazard to arise on Unix.

• Unix emulation: The difference between the FD/handle-passing models makes it tricky to emulate Unix domain sockets on Windows.

  An accurate emulation would need to implement the semantics of how an FD object can be stored in the message buffer of a socket that might be read from concurrently by multiple processes.

  Cygwin implements an emulation of Unix domain sockets on Windows, but it currently does not implement FD-passing.

(Updated on 2015/07/03 [previous version]: Corrected the post to say that Cygwin does not implement FD-passing.)

Can cached memory accesses do double-sided row hammering?

There are indications that it is possible to cause bit flips in memory by row hammering without using CLFLUSH, using normal cached memory accesses.

This makes me wonder: Is it possible to do double-sided row hammering using cached memory accesses, or only single-sided row hammering? The former is more likely to cause bit flips, and might be the only way to cause bit flips on some machines, such as those using a 2x refresh rate -- i.e. those configured to refresh DRAM every 32ms instead of every 64ms. (See the rowhammer blog post for more background.)

The answer appears to be "no" -- at least on my test machine.

For this machine (which has a Sandy Bridge CPU), I figured out how physical addresses map to cache sets and to banks and rows in DRAM. We can use these mappings to answer questions about what kinds of row hammering are possible using cached memory accesses.

More specifically, my question is this: For a machine with an N-way L3 cache, is it possible to pick N+1 addresses that map to the same cache set, where at least two of these addresses map to rows R-1 and R+1 in one bank (for some neighbouring row R)? If so, repeatedly accessing these addresses would cause cache misses that cause rows R-1 and R+1 to be repeatedly activated. That puts more stress on row R (the victim row) than repeatedly activating only row R-1 or row R+1.

The answer to this is "no": It's not possible to pick two such physical addresses.

Here's why:

Suppose we have two such addresses, A and B. Then:

• The addresses map to the same bank, so:
  (1): A[14:17] ^ A[18:21] = B[14:17] ^ B[18:21]
  (using the bank/row XOR scheme I described previously)

• The addresses are 2 rows apart, so:
  (2): A[18:32] + 2 = B[18:32]

• The addresses map to the same cache set, so:
  (3): A[6:17] = B[6:17]
  (also, SliceHash(A[17:32]) = SliceHash(B[17:32]), but we don't need this property)

(2) implies that A[19] = ~B[19].

(3) implies that A[14:17] = B[14:17]. Combining that with (1) gives A[18:21] = B[18:21]. That implies A[19] = B[19], which contradicts (2), so the constraints can't be satisfied.

Basically, the bank/row XOR scheme used by the memory controller's address mapping gets in the way.

It looks like an attacker who is trying to do row hammering using only cached memory accesses (e.g. from Javascript) would, on this particular machine, have to try one of two routes:

• Do single-sided row hammering only.
• Attempt double-sided hammering by picking two sets of 13 addresses (given a 12-way L3 cache), mapping to two different cache sets. This would likely halve the rate of memory accesses (relative to accessing one set of 13 addresses).

Some further notes

I might be getting ahead of myself by doing this analysis, because the feasibility of causing rowhammer-induced bit flips via cached accesses has not yet been demonstrated on a wide selection of machines.

Also, I am asking whether double-sided hammering is possible, not practical, given the constraint that the attacker's memory accesses must go through the cache. While my analysis shows that the risk of causing double-sided hammering is zero with this constraint, the risk might only be negligible without this constraint. Even if a program can use CLFLUSH to bypass the cache, it doesn't necessarily have a way to determine which pairs of addresses map to rows that are spaced apart by two (see the rowhammer blog post for further discussion). Such a program might only be able to get such pairs by chance, by selecting addresses randomly, which would take a long time.

Lastly, my analysis makes assumptions about a machine's DRAM and L3 cache mappings. It would need to be redone for other machines where these assumptions aren't true.

How physical addresses map to rows and banks in DRAM

In my previous blog post, I discussed how Intel Sandy Bridge CPUs map physical addresses to locations in the L3 cache.

Now I'll discuss how these CPUs' memory controllers map physical addresses to locations in DRAM -- specifically, to row, bank and column numbers in DRAM modules. Let's call this the DRAM address mapping. I'll use one test machine as a case study.

Motivation: the rowhammer bug

I am interested in the DRAM address mapping because it is relevant to the "rowhammer" bug.

Rowhammer is a problem with some DRAM modules whereby certain pessimal memory access patterns can cause memory corruption. In these DRAMs, repeatedly activating a row of memory (termed "row hammering") can produce electrical disturbances that produce bit flips in vulnerable cells in adjacent rows of memory.

These repeated row activations can be caused by repeatedly accessing a pair of DRAM locations that are in different rows of the same bank of DRAM. Knowing the DRAM address mapping is useful because it tells us which pairs of addresses satisfy this "same bank, different row" (SBDR) property.

Guessing and checking an address mapping

For my case study, I have a test machine containing DRAM that is vulnerable to the rowhammer problem. Running rowhammer_test on this machine demonstrates bit flips.

I'd like to know what the DRAM address mapping is for this machine, but apparently it isn't publicly documented: This machine has a Sandy Bridge CPU, but Intel don't document the address mapping used by these CPUs' memory controllers.

rowhammer_test does not actually need to identify SBDR address pairs. rowhammer_test just repeatedly tries hammering randomly chosen address pairs. Typically 1/8 or 1/16 of these pairs will be SBDR pairs, because our machine has 8 banks per DIMM (and 16 banks in total). So, while we don't need to know the DRAM address mapping to cause bit flips on this machine, knowing it would help us be more targeted in our testing.

Though the address mapping isn't documented, I found that I can make an educated guess at what the mapping is, based on the DRAM's geometry, and then verify the guess based on the physical addresses that rowhammer_test reports. rowhammer_test can report the physical addresses where bit flips occur ("victims") and the pairs of physical addresses that produce those bit flips ("aggressors"). Since these pairs must be SBDR pairs, we can check a hypothesised address mapping against this empirical data.

Memory geometry

The first step in hypothesising an address mapping for a machine is to check how many DIMMs the machine has and how these DIMMs are organised internally.

I can query information about the DIMMs using the decode-dimms tool on Linux. (In Ubuntu, decode-dimms is in the i2c-tools package.) This tool decodes the DIMMs' SPD (Serial Presence Detect) metadata.

My test machine has 2 * 4GB SO-DIMMs, giving 8GB of memory in total.

decode-dimms reports the following information for both of the DIMMs:

Size                                            4096 MB
Banks x Rows x Columns x Bits                   8 x 15 x 10 x 64
Ranks                                           2

This means that, for each DIMM:

• Each of the DIMM's banks contains 2^15 rows (32768 rows).
• Each row contains 2^10 * 64 bits = 2^16 bits = 2^13 bytes = 8 kbytes.

Each DIMM has 2 ranks and 8 banks. Cross checking the capacity of the DIMM gives us the reported size, as expected:

8 kbytes per row * 32768 rows * 2 ranks * 8 banks = 4096 MB = 4 GB

The DRAM address mapping

On my test machine, it appears that the bits of physical addresses are used as follows:

• Bits 0-5: These are the lower 6 bits of the byte index within a row (i.e. the 6-bit index into a 64-byte cache line).
• Bit 6: This is a 1-bit channel number, which selects between the 2 DIMMs.
• Bits 7-13: These are the upper 7 bits of the index within a row (i.e. the upper bits of the column number).
• Bits 14-16: These are XOR'd with the bottom 3 bits of the row number to give the 3-bit bank number.
• Bit 17: This is a 1-bit rank number, which selects between the 2 ranks of a DIMM (which are typically the two sides of the DIMM's circuit board).
• Bits 18-32: These are the 15-bit row number.
• Bits 33+: These may be set because physical memory starts at physical addresses greater than 0.

Why is the mapping like that?

This mapping fits the results from rowhammer_test (see below), but we can also explain that the address bits are mapped this way to give good performance for typical memory access patterns, such as sequential accesses and strided accesses:

• Channel parallelism: Placing the channel number at bit 6 means that cache lines will alternate between the two channels (i.e. the two DIMMs), which can be accessed in parallel. This means that if we're accessing addresses sequentially, the load will be spread across the two channels.

  As an aside, Ivy Bridge (the successor to Sandy Bridge) apparently makes the mapping of the channel number more complex. An Intel presentation mentions "Channel hashing" and says that this "Allows channel selection to be made based on multiple address bits. Historically, it had been "A[6]". Allows more even distribution of memory accesses across channels."

• Bank thrashing: Generally, column, bank and row numbers are arranged to minimise "bank thrashing" (frequently changing a bank's currently activated row).

  Some background: DRAM modules are organised into banks, which in turn are organised into rows. Each bank has a "currently activated row" whose contents are copied into a row buffer which acts as a cache that can be accessed quickly. Accessing a different row takes longer because that row must be activated first. So, the DRAM address mapping places SBDR pairs as far apart as possible in physical address space.

  Row hammering is a special case of bank thrashing where two particular rows are repeatedly activated (perhaps deliberately).

• Bank parallelism: Banks can be accessed in parallel (though to a lesser degree than channels), so the bank number changes before the row number as the address is increased.

• XOR scheme: XORing the row number's lower bits into the bank number is a trick to avoid bank thrashing when accessing arrays with large strides. For example, in the mapping above, the XORing causes addresses X and X+256k to be placed in different banks instead of being an SBDR pair.

  Bank/row XORing schemes are described in various places, such as:

  • David Tawei Wang's PhD thesis, "Modern DRAM memory systems: Performance analysis and scheduling algorithms", 2005. See section 5.3.5, "Bank Address Aliasing (stride collision)". This thesis has some good background info on DRAM in general.
  • The paper "Reducing DRAM Latencies with an Integrated Memory Hierarchy Design", 2001. See Figure 3.

Checking against rowhammer_test's output

I ran rowhammer_test_ext (the extended version of rowhammer_test) on my test machine for 6 hours, and it found repeatable bit flips at 22 locations. (See the raw data and analysis code.)

The row hammering test generates a set of (A1, A2, V) triples, where:

• V is the victim address where we see the bit flip.
• A1 and A2 are the aggressor addresses that we hammer.
• We sort A1 and A2 so that A1 is closer to V than A2 is. We tentatively assume that the closer address, A1, is the one that actually causes the bit flips (though this wouldn't necessarily be true if the DRAM address mapping were more complicated).

There are three properties we expect to hold for all of these results:

• Row: A1 and V's row numbers should differ by 1 -- i.e. they should be in adjacent rows. (A2 can have any row number.)

  This property makes it easy to work out where the bottom bits of the row number are in the physical address.

  We find this property holds for all but 2 of the results. In those 2 results, the row numbers differ by 3 rather than 1.

• Bank: V, A1 and A2 should have the same bank number. Indeed, we find this property holds for all 22 results. This only holds when applying the row/bank XORing scheme.

• Channel: V, A1 and A2 should have the same channel number. This holds for all the results. It happens that all of our results have channel=0, because rowhammer_test only selects 4k-aligned addresses and so only tests one channel. (Maybe this could be considered a bug.)

Possible further testing

There are two further experiments we could run to check whether the DRAM address mapping evaluates the SBDR property correctly, which I haven't tried yet:

• Timing tests: Accessing SBDR address pairs repeatedly should be slower than accessing non-SBDR pairs repeatedly, because the former cause row activations and the latter don't.

• Exhaustive rowhammer testing: Once we've found an aggressor address, A1, that causes a repeatable bit flip, we can test this against many values of address A2. Hammering (A1, A2) can produce bit flips only if this is an SBDR pair.

Furthermore, taking out one DIMM from our test machine should remove the channel bit from the DRAM address mapping and change the aggressor/victim addresses accordingly. We could check whether this is the case.