[SRU B][PATCH 0/1] kernel panic using CIFS share in smb2_push_mandatory_locks()

Guilherme G. Piccoli gpiccoli at canonical.com
Wed Jul 17 20:02:35 UTC 2019

BugLink: http://bugs.launchpad.net/bugs/1795659


* We got reports of a kernel crash in cifs module with the following signature:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
IP: smb2_push_mandatory_locks+0x10e/0x3b0 [cifs]
PGD 0 P4D 0
RIP: 0010:smb2_push_mandatory_locks+0x10e/0x3b0 [cifs]
Call Trace:
 cifs_oplock_break+0x12f/0x3d0 [cifs]

* Low-level analysis (decodecode script output and the objdump of the function)
revealed that we are crashing in a NULL ptr dereference when trying to access
"cfile->tlink"; below, a snippet of the objdump at function smb2_push_mandatory_locks():

mov 0x10(%r14),%r15 # %r15 = cifsFileInfo *cfile
mov 0x18(%r14),%rbx # %rbx = cifsLockInfo *li = (fdlocks->locks)
lea 0x18(%r14),%r12
mov 0x90(%r15),%rax # %rax = struct tcon_link *tlink (cfile->tlink)
cmp %r12,%rbx
mov 0x38(%rax),%rax # <--- TRAP [trying to get cifs_tcon *tl_tcon]

* After discussing the issue with CIFS maintainers (Steve French and Pavel
Shilovsky) they suggested commit b98749cac4a6 ("CIFS: keep FileInfo handle live
during oplock break") [http://git.kernel.org/linus/b98749cac4a6] as a fix for
multiple reports of this kind of crash.

* The fix was sent to stable kernels and is present in Ubuntu kernels 5.0 and newer.
We are requesting the SRU for this patch here in order to fix the crashes, after
reports of successful testing with the patch (see below section) and since the
patch is restricted to the cifs module scope and accepted on linux stable.

* Alternatively the issue is known to be avoided when oplocks are disabled using
"cifs.enable_oplocks=N" module parameter.

[Test case]

* Unfortunately we cannot reproduce the issue. The patch proposed here was
validated by us with xfstests (instructions followed from
https://wiki.samba.org/index.php/Xfstesting-cifs) and fio. Also, we
have a user report of test validation using LISA (https://github.com/LIS/LISAv2).

* Using xfstest with the exclusions proposed in the link above we managed to get
the same results as a non-patched kernel, i.e., the same tests failed in both
kernels, we didn't get worse results with the patch. Fio also didn't show
noticeable performance regression with the patch.

[Regression potential]

* The patch was validated by the cifs filesystem maintainers (in fact they
suggested its inclusion in Ubuntu) and by the aforementioned tests; also,
the scope is restricted to cifs only so the likelihood of regressions is considered low.

* Due to the nature of the code modification (add a new reference of a file
handler and manipulate it in different places), I consider that if we have a
regression it'll manifest as deadlock/blocked tasks, not something more serious
like crashes or data corruption.

Aurelien Aptel (1):
  CIFS: keep FileInfo handle live during oplock break

 fs/cifs/cifsglob.h |  2 ++
 fs/cifs/file.c     | 30 +++++++++++++++++++++++++-----
 fs/cifs/misc.c     | 25 +++++++++++++++++++++++--
 fs/cifs/smb2misc.c |  6 +++---
 4 files changed, 53 insertions(+), 10 deletions(-)


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