Re: [PATCH v2] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread

[Christian Schoenebeck]

On Freitag, 28. Januar 2022 23:33:26 CET Vitaly Chikunov wrote:
> `struct dirent' returned from readdir(3) could be shorter than
> `sizeof(struct dirent)', thus memcpy of sizeof length will overread
> into unallocated page causing SIGSEGV. Example stack trace:
> 
>  #0  0x00005555559ebeed v9fs_co_readdir_many (/usr/bin/qemu-system-x86_64 +
> 0x497eed) #1  0x00005555559ec2e9 v9fs_readdir (/usr/bin/qemu-system-x86_64
> + 0x4982e9) #2  0x0000555555eb7983 coroutine_trampoline
> (/usr/bin/qemu-system-x86_64 + 0x963983) #3  0x00007ffff73e0be0 n/a (n/a +
> 0x0)
> 
> While fixing, provide a helper for any future `struct dirent' cloning.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841
> Cc: qemu-stable@nongnu.org
> Co-authored-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> ---
> Tested on x86-64 Linux.

I was too optimistic. Looks like this needs more work. With this patch applied
the 9p test cases [1] are crashing now:

$ gdb --args tests/qtest/qos-test -m slow
...
# Start of flush tests
ok 50 
/x86_64/pc/i440FX-pcihost/pci-bus-pc/pci-bus/virtio-9p-pci/virtio-9p/virtio-9p-tests/synth/flush/success
ok 51 
/x86_64/pc/i440FX-pcihost/pci-bus-pc/pci-bus/virtio-9p-pci/virtio-9p/virtio-9p-tests/synth/flush/ignored
# End of flush tests
# Start of readdir tests
Broken pipe

Thread 1 "qos-test" received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007ffff7b7d537 in __GI_abort () at abort.c:79
#2  0x00005555555ba495 in qtest_client_socket_recv_line (s=0x5555557663c0) at 
../tests/qtest/libqtest.c:503
#3  0x00005555555ba5b3 in qtest_rsp_args (s=0x5555557663c0, expected_args=2) at 
../tests/qtest/libqtest.c:523
#4  0x00005555555bbdb4 in qtest_clock_rsp (s=0x5555557663c0) at 
../tests/qtest/libqtest.c:970
#5  0x00005555555bbe55 in qtest_clock_step (s=0x5555557663c0, step=100) at 
../tests/qtest/libqtest.c:985
#6  0x00005555555cdc21 in qvirtio_wait_used_elem (qts=0x5555557663c0, 
d=0x555555779b48, vq=0x5555557b0480, desc_idx=8, len=0x0, timeout_us=10000000)
    at ../tests/qtest/libqos/virtio.c:220
#7  0x00005555555ae79f in v9fs_req_wait_for_reply (req=0x5555557899a0, len=0x0) 
at ../tests/qtest/virtio-9p-test.c:278
#8  0x00005555555b03bf in fs_readdir (obj=0x555555779bb0, data=0x0, 
t_alloc=0x5555557448b8) at ../tests/qtest/virtio-9p-test.c:851
#9  0x00005555555990c4 in run_one_test (arg=0x5555557ac600) at 
../tests/qtest/qos-test.c:182
#10 0x00007ffff7f02b9e in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#11 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#12 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#13 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#14 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#15 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#16 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#17 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#18 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#19 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#20 0x00007ffff7f0299b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#21 0x00007ffff7f0308a in g_test_run_suite () from 
/lib/x86_64-linux-gnu/libglib-2.0.so.0
#22 0x00007ffff7f030a1 in g_test_run () from 
/lib/x86_64-linux-gnu/libglib-2.0.so.0
#23 0x00005555555995a3 in main (argc=1, argv=0x7fffffffe508, 
envp=0x7fffffffe528) at ../tests/qtest/qos-test.c:338
(gdb)

[1] https://wiki.qemu.org/Documentation/9p#Test_Cases

Best regards,
Christian Schoenebeck