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Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct di
From: |
Greg Kurz |
Subject: |
Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread |
Date: |
Fri, 4 Feb 2022 16:16:06 +0100 |
On Fri, 04 Feb 2022 15:12:18 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> On Freitag, 4. Februar 2022 14:55:45 CET Philippe Mathieu-Daudé via wrote:
> > On 4/2/22 06:06, Vitaly Chikunov wrote:
> > > `struct dirent' returned from readdir(3) could be shorter (or longer)
> > > 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 again.
> > >
> > > Changes from v2:
> > > - Make it work with a simulated dirent where d_reclen is 0, which was
> > >
> > > caused abort in readdir qos-test, by using fallback at runtime.
> > >
> > > hw/9pfs/codir.c | 3 +--
> > > include/qemu/osdep.h | 13 +++++++++++++
> > > util/osdep.c | 18 ++++++++++++++++++
> > > 3 files changed, 32 insertions(+), 2 deletions(-)
> > >
> > > +struct dirent *
> > > +qemu_dirent_dup(struct dirent *dent)
> > > +{
> > > + size_t sz = 0;
> > > +#if defined _DIRENT_HAVE_D_RECLEN
> > > + /* Avoid use of strlen() if there's d_reclen. */
> > > + sz = dent->d_reclen;
> > > +#endif
> > > + if (sz == 0) {
> >
> > If _DIRENT_HAVE_D_RECLEN is defined, this case is unlikely...
> >
> > > + /* Fallback to the most portable way. */
> > > + sz = offsetof(struct dirent, d_name) +
> > > + strlen(dent->d_name) + 1;
> > > + }
> > > + struct dirent *dst = g_malloc(sz);
> > > + return memcpy(dst, dent, sz);
> > > +}
> >
> > What about this?
> >
> > struct dirent *
> > qemu_dirent_dup(struct dirent *dent)
> > {
> > size_t sz;
> >
> > #if defined _DIRENT_HAVE_D_RECLEN
> > /* Avoid use of strlen() if there's d_reclen. */
> > sz = dent->d_reclen;
> > #else
> > /* Fallback to the most portable way. */
> > sz = offsetof(struct dirent, d_name) +
> > strlen(dent->d_name) + 1;
> > #endif
> >
> > return g_memdup(dent, sz);
> > }
>
> That was the intentional change v2 -> v3 Philippe. Previous v2 crashed the
> 9p 'synth' tests:
>
> https://lore.kernel.org/qemu-devel/2627488.0S70g7mNYN@silver/T/#ma09bedde59a91e6435443e151f7e49fef8616e4c
>
> You might argue that the 9p 'synth' driver should instead populate d_reclen
> instead, but this v3 is a simpler solution and guarantees to always work. So
> I'd prefer to go with Vitaly's v3 for now, especially as this patch would go
> to the stable branches as well.
>
This really is a band-aid. Anyone reading this will react as Philippe,
and this is common code, not just 9pfs. With correctly populated dirents,
the helper could be as simple as:
struct dirent *
qemu_dirent_dup(struct dirent *dent)
{
size_t sz = offsetof(struct dirent, d_name) + _D_EXACT_NAMLEN(dent) + 1;
return g_memdup(dent, sz);
}
If this is a cycles problem, I can help reviewing the fixes on
the synth driver, or alternatively you can keep this code
somewhere under 9pfs but please don't put that in common utils.
Cheers,
--
Greg
> Best regards,
> Christian Schoenebeck
>
>
- [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Vitaly Chikunov, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Christian Schoenebeck, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Dmitry V. Levin, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Philippe Mathieu-Daudé, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Christian Schoenebeck, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread,
Greg Kurz <=
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Vitaly Chikunov, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Dmitry V. Levin, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Philippe Mathieu-Daudé, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Christian Schoenebeck, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Philippe Mathieu-Daudé, 2022/02/04
- Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Christian Schoenebeck, 2022/02/04
Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Dmitry V. Levin, 2022/02/04
Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread, Christian Schoenebeck, 2022/02/05