On 1/6/26 23:04, Matthew Lugg wrote:
On 11/17/25 17:09, Matthew Lugg wrote:
This version of the series should address all feedback I received. The original
cover letter is replicated below.
I was recently debugging a strange crash in a downstream project which turned
out to be a QEMU bug related to the `mremap` implementation in linux-user. In
practice, this bug essentially led to arbitrary memory regions being unmapped
when a 32-bit guest, running on a 64-bit host, uses `mremap` to shrink a memory
mapping.
The first patch in this set resolves that bug. Since the patch is very simple,
and the bug is quite likely to be hit, I suspect that that commit is a good
candidate for qemu-stable.
The following two patches just resolve two more bugs I became aware of whilst
working on this code. I believe the messages in those patches contain all
necessary context. They are less critical and the fixes more complex, so are
likely not suitable for backporting into qemu-stable.
The final commits adds tcg tests for the fixed `mremap` behavior. The third fix
is unfortunately difficult to test programmatically, but I have confirmed that
it behaves as expected by observing the output of `strace qemu-i386 repro`,
where `repro` is the following C program:
#define _GNU_SOURCE
#include <stddef.h>
#include <sys/mman.h>
int main(void) {
char *a = mmap(NULL, 4097, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS,
-1, 0);
char *b = mmap(NULL, 4097, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS,
-1, 0);
mremap(b, 4097, 4097, MREMAP_FIXED | MREMAP_MAYMOVE, a);
// QEMU has now leaked a page of its memory reservation!
return 0;
}
Prior to the patch, as the comment says, QEMU leaks a page of its address space
reservation (i.e. the page becomes unmapped). After the patch, QEMU correctly
reclaims that page with `mmap`.
Matthew Lugg (4):
linux-user: fix mremap unmapping adjacent region
linux-user: fix mremap errors for invalid ranges
linux-user: fix reserved_va page leak in do_munmap
tests: add tcg coverage for fixed mremap bugs
linux-user/mmap.c | 16 ++++++++-----
tests/tcg/multiarch/test-mmap.c | 42 +++++++++++++++++++++++++++++++--
2 files changed, 50 insertions(+), 8 deletions(-)
Hey everyone, just bumping this series since I suspect it got lost. It should be fairly
straightforward to get this in: patches 1 and 3 were already approved here, and 2 and 4
only have small changes since v1 to account for feedback.
Let me know if anything else is needed on my end---and happy new year!
Queued, thanks.
r~
