[PATCH] mount.cifs: Fix double-free issue when mounting with setuid root

Aurélien Aptel aaptel at suse.com
Mon Sep 9 12:46:50 UTC 2019

" Paulo Alcantara (SUSE) " <paulo at paulo.ac> writes:
> It can be easily reproduced with the following:
>   # chmod +s `which mount.cifs`
>   # echo "//localhost/share /mnt cifs \
>     users,username=foo,password=XXXX" >> /etc/fstab
>   # su - foo
>   $ mount /mnt
>   free(): double free detected in tcache 2
>   Child process terminated abnormally.
> The problem was that check_fstab() already freed orgoptions pointer
> and then we freed it again in main() function.
> Fixes: bf7f48f4c7dc ("mount.cifs.c: fix memory leaks in main func")
> Signed-off-by: Paulo Alcantara (SUSE) <paulo at paulo.ac>

Reviewed-by: Aurelien Aptel <aaptel at suse.com>

I've compiled next branch with ASAN and can confirm the double-free and
the fix works

Compiling with ASAN

$ CFLAGS=-fsanitize=address \
  LDFLAGS=-fsanitize=address \
  ac_cv_func_malloc_0_nonnull=yes \
$ make clean && make -j4  

Next branch

$ mount /mnt; echo $?
==29883==ERROR: AddressSanitizer: attempting double-free on 0x607000000020 in thread T0:
    #0 0x7f69d480a1b8 in __interceptor_free (/usr/lib64/libasan.so.4+0xdc1b8)
    #1 0x559381795f33 in main (/sbin/mount.cifs+0xef33)
    #2 0x7f69d4394f89 in __libc_start_main (/lib64/libc.so.6+0x20f89)
    #3 0x55938178e079 in _start (/sbin/mount.cifs+0x7079)

0x607000000020 is located 0 bytes inside of 68-byte region [0x607000000020,0x607000000064)
freed by thread T0 here:
    #0 0x7f69d480a1b8 in __interceptor_free (/usr/lib64/libasan.so.4+0xdc1b8)
    #1 0x55938178e372 in check_fstab (/sbin/mount.cifs+0x7372)
    #2 0x559381794661 in assemble_mountinfo (/sbin/mount.cifs+0xd661)
    #3 0x559381795eef in main (/sbin/mount.cifs+0xeeef)
    #4 0x7f69d4394f89 in __libc_start_main (/lib64/libc.so.6+0x20f89)

previously allocated by thread T0 here:
    #0 0x7f69d480a510 in malloc (/usr/lib64/libasan.so.4+0xdc510)
    #1 0x7f69d43fc2a9 in __GI___strndup (/lib64/libc.so.6+0x882a9)

SUMMARY: AddressSanitizer: double-free (/usr/lib64/libasan.so.4+0xdc1b8) in __interceptor_free

With fix

$ mount /mnt; echo $?

Aurélien Aptel / SUSE Labs Samba Team
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