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author | Max Reitz <mreitz@redhat.com> | 2020-06-08 11:31:11 +0200 |
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committer | Dr. David Alan Gilbert <dgilbert@redhat.com> | 2020-06-17 17:48:38 +0100 |
commit | 63659fe74e76f5c5285466f0c5cfbdca65b3688e (patch) | |
tree | 6fe68802a1fa9dae71813b960ee663fc23762cd0 /qom | |
parent | 26bf4a29213b432eb390726c698a1915550a9cf9 (diff) | |
download | qemu-63659fe74e76f5c5285466f0c5cfbdca65b3688e.zip qemu-63659fe74e76f5c5285466f0c5cfbdca65b3688e.tar.gz qemu-63659fe74e76f5c5285466f0c5cfbdca65b3688e.tar.bz2 |
virtiofsd: Whitelist fchmod
lo_setattr() invokes fchmod() in a rarely used code path, so it should
be whitelisted or virtiofsd will crash with EBADSYS.
Said code path can be triggered for example as follows:
On the host, in the shared directory, create a file with the sticky bit
set and a security.capability xattr:
(1) # touch foo
(2) # chmod u+s foo
(3) # setcap '' foo
Then in the guest let some process truncate that file after it has
dropped all of its capabilities (at least CAP_FSETID):
int main(int argc, char *argv[])
{
capng_setpid(getpid());
capng_clear(CAPNG_SELECT_BOTH);
capng_updatev(CAPNG_ADD, CAPNG_PERMITTED | CAPNG_EFFECTIVE, 0);
capng_apply(CAPNG_SELECT_BOTH);
ftruncate(open(argv[1], O_RDWR), 0);
}
This will cause the guest kernel to drop the sticky bit (i.e. perform a
mode change) as part of the truncate (where FATTR_FH is set), and that
will cause virtiofsd to invoke fchmod() instead of fchmodat().
(A similar configuration exists further below with futimens() vs.
utimensat(), but the former is not a syscall but just a wrapper for the
latter, so no further whitelisting is required.)
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1842667
Reported-by: Qian Cai <caiqian@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200608093111.14942-1-mreitz@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Diffstat (limited to 'qom')
0 files changed, 0 insertions, 0 deletions