Re: [PATCH] security: smack: fix out-of-bounds access in smk_parse_smack()

[Casey Schaufler]

On 11/8/2014 6:48 AM, Andrey Ryabinin wrote:
> From: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
>
> Setting smack label on file (e.g. 'attr -S -s SMACK64 -V "test" test')
> triggered following spew on the kernel with KASan applied:
>     ==================================================================
>     BUG: AddressSanitizer: out of bounds access in strncpy+0x28/0x60 at addr ffff8800059ad064
>     =============================================================================
>     BUG kmalloc-8 (Not tainted): kasan error
>     -----------------------------------------------------------------------------
>
>     Disabling lock debugging due to kernel taint
>     INFO: Slab 0xffffea0000166b40 objects=128 used=7 fp=0xffff8800059ad080 flags=0x4000000000000080
>     INFO: Object 0xffff8800059ad060 @offset=96 fp=0xffff8800059ad080
>
>     Bytes b4 ffff8800059ad050: a0 df 9a 05 00 88 ff ff 5a 5a 5a 5a 5a 5a 5a 5a  ........ZZZZZZZZ
>     Object ffff8800059ad060: 74 65 73 74 6b 6b 6b a5                          testkkk.
>     Redzone ffff8800059ad068: cc cc cc cc cc cc cc cc                          ........
>     Padding ffff8800059ad078: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ
>     CPU: 0 PID: 528 Comm: attr Tainted: G    B          3.18.0-rc1-mm1+ #5
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>      0000000000000000 ffff8800059ad064 ffffffff81534cf2 ffff880005a5bc40
>      ffffffff8112fe1a 0000000100800006 0000000f059ad060 ffff880006000f90
>      0000000000000296 ffffea0000166b40 ffffffff8107ca97 ffff880005891060
>     Call Trace:
>     ? dump_stack (lib/dump_stack.c:52)
>     ? kasan_report_error (mm/kasan/report.c:102 mm/kasan/report.c:178)
>     ? preempt_count_sub (kernel/sched/core.c:2651)
>     ? __asan_load1 (mm/kasan/kasan.h:50 mm/kasan/kasan.c:248 mm/kasan/kasan.c:358)
>     ? strncpy (lib/string.c:121)
>     ? strncpy (lib/string.c:121)
>     ? smk_parse_smack (security/smack/smack_access.c:457)
>     ? setxattr (fs/xattr.c:343)
>     ? smk_import_entry (security/smack/smack_access.c:514)
>     ? smack_inode_setxattr (security/smack/smack_lsm.c:1093 (discriminator 1))
>     ? security_inode_setxattr (security/security.c:602)
>     ? vfs_setxattr (fs/xattr.c:134)
>     ? setxattr (fs/xattr.c:343)
>     ? setxattr (fs/xattr.c:360)
>     ? get_parent_ip (kernel/sched/core.c:2606)
>     ? preempt_count_sub (kernel/sched/core.c:2651)
>     ? __percpu_counter_add (arch/x86/include/asm/preempt.h:98 lib/percpu_counter.c:90)
>     ? get_parent_ip (kernel/sched/core.c:2606)
>     ? preempt_count_sub (kernel/sched/core.c:2651)
>     ? __mnt_want_write (arch/x86/include/asm/preempt.h:98 fs/namespace.c:359)
>     ? path_setxattr (fs/xattr.c:380)
>     ? SyS_lsetxattr (fs/xattr.c:397)
>     ? system_call_fastpath (arch/x86/kernel/entry_64.S:423)
>     Read of size 1 by task attr:
>     Memory state around the buggy address:
>      ffff8800059ace80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>      ffff8800059acf00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>      ffff8800059acf80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     >ffff8800059ad000: 00 fc fc fc 00 fc fc fc 05 fc fc fc 04 fc fc fc
>                                                            ^
>      ffff8800059ad080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>      ffff8800059ad100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>      ffff8800059ad180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>     ==================================================================
>
> strncpy() copies one byte more than the source string has.
> Fix this by passing the correct length to strncpy().
>
> Now we can remove initialization of the last byte in 'smack' string
> because kzalloc() already did this for us.
>
> Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>

Applied to git://git.gitorious.org/smack-next/kernel.git#smack-for-3.19

> ---
>  security/smack/smack_access.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/security/smack/smack_access.c b/security/smack/smack_access.c
> index 5b970ff..ad75ddf 100644
> --- a/security/smack/smack_access.c
> +++ b/security/smack/smack_access.c
> @@ -452,10 +452,9 @@ char *smk_parse_smack(const char *string, int len)
>  		return NULL;
>  
>  	smack = kzalloc(i + 1, GFP_KERNEL);
> -	if (smack != NULL) {
> -		strncpy(smack, string, i + 1);
> -		smack[i] = '\0';
> -	}
> +	if (smack != NULL)
> +		strncpy(smack, string, i);
> +
>  	return smack;
>  }
>  

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