Re: [PATCH 2/3] Smack: handle zero-length security labels without panic

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Konstantin Khlebnikov (k.khlebnikov@samsung.com):
> Zero-length security labels are invalid but kernel should handle them.
> 
> This patch fixes kernel panic after setting zero-length security labels:
> # attr -S -s "SMACK64" -V "" file
> 
> And after writing zero-length string into smackfs files syslog and onlycp:
> # python -c 'import os; os.write(1, "")' > /smack/syslog
> 
> The problem is caused by brain-damaged logic in function smk_parse_smack()
> which takes pointer to buffer and its length but if length below or equal zero
> it thinks that the buffer is zero-terminated. Unfortunately callers of this
> function are widely used and proper fix requires serious refactoring.
> 
> Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com>

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

> 
> ---
> 
> Example:
> 
> [   28.063935] BUG: unable to handle kernel NULL pointer dereference at           (null)
> [   28.064623] IP: [<ffffffff812ea780>] strlen+0x0/0x30
> [   28.064623] PGD 29a77067 PUD 2972d067 PMD 0
> [   28.064623] Oops: 0000 [#1] SMP
> [   28.064623] Modules linked in:
> [   28.064623] CPU: 0 PID: 824 Comm: attr Not tainted 3.16.0+ #6
> [   28.064623] task: ffff880029bd88c0 ti: ffff8800297d0000 task.ti: ffff8800297d0000
> [   28.064623] RIP: 0010:[<ffffffff812ea780>]  [<ffffffff812ea780>] strlen+0x0/0x30
> [   28.064623] RSP: 0018:ffff8800297d3ca8  EFLAGS: 00010246
> [   28.064623] RAX: 0000000000000001 RBX: ffff880029c096c0 RCX: 0000000000000001
> [   28.064623] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> [   28.064623] RBP: ffff8800297d3cd0 R08: ffffffff81e3d940 R09: ffff88002a3e8588
> [   28.064623] R10: 8080808080808080 R11: 0000000000000000 R12: 0000000000000000
> [   28.064623] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> [   28.064623] FS:  00007effe391a700(0000) GS:ffff88002a400000(0000) knlGS:0000000000000000
> [   28.064623] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   28.064623] CR2: 0000000000000000 CR3: 00000000297a1000 CR4: 00000000001406b0
> [   28.064623] Stack:
> [   28.064623]  ffffffff812a6fa5 ffff880029c096c0 0000000000000001 0000000000000000
> [   28.064623]  0000000000000000 ffff8800297d3cf8 ffffffff812a70ae ffff880029c096c0
> [   28.064623]  0000000000000001 0000000000000000 ffff8800297d3d88 ffffffff812a64dc
> [   28.064623] Call Trace:
> [   28.064623]  [<ffffffff812a6fa5>] ? smk_parse_smack+0xe5/0x110
> [   28.064623]  [<ffffffff812a70ae>] smk_import_entry+0xe/0x130
> [   28.064623]  [<ffffffff812a64dc>] smack_inode_setxattr+0x11c/0x280
> [   28.064623]  [<ffffffff8116a767>] ? lookup_fast+0x177/0x2e0
> [   28.064623]  [<ffffffff81177001>] ? dput+0x21/0x1a0
> [   28.064623]  [<ffffffff812a0eeb>] security_inode_setxattr+0x1b/0x30
> [   28.064623]  [<ffffffff8118488f>] vfs_setxattr+0x6f/0xb0
> [   28.064623]  [<ffffffff81184a76>] setxattr+0x1a6/0x1f0
> [   28.064623]  [<ffffffff8116af6d>] ? final_putname+0x1d/0x40
> [   28.064623]  [<ffffffff8116b1c4>] ? putname+0x24/0x40
> [   28.064623]  [<ffffffff8117144a>] ? user_path_at_empty+0x5a/0xa0
> [   28.064623]  [<ffffffff81162b84>] ? __sb_start_write+0x44/0xe0
> [   28.064623]  [<ffffffff8113cd21>] ? do_brk+0x241/0x320
> [   28.064623]  [<ffffffff81184d97>] SyS_lsetxattr+0x87/0xe0
> [   28.064623]  [<ffffffff81806f69>] system_call_fastpath+0x16/0x1b
> [   28.064623] Code: 89 f8 48 89 e5 f6 82 40 3e a6 81 20 74 15 0f 1f 44 00 00 48 83 c0 01 0f b6 10 f6 82 40 3e a6 81 20 75 f0 5d c3 66 0f 1f 44 00 00 <80> 3f 00 55 48 89 e5 74 15 48 89 f8 0f 1f 40 00 48 83 c0 01 80
> [   28.064623] RIP  [<ffffffff812ea780>] strlen+0x0/0x30
> [   28.064623]  RSP <ffff8800297d3ca8>
> [   28.064623] CR2: 0000000000000000
> [   28.093561] ---[ end trace de1055429a98a5be ]---
> ---
>  security/smack/smack_lsm.c |    2 +-
>  security/smack/smackfs.c   |    4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index b11ab23..afa5ad0 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -923,7 +923,7 @@ static int smack_inode_setxattr(struct dentry *dentry, const char *name,
>  		rc = -EPERM;
>  
>  	if (rc == 0 && check_import) {
> -		skp = smk_import_entry(value, size);
> +		skp = size ? smk_import_entry(value, size) : NULL;
>  		if (skp == NULL || (check_star &&
>  		    (skp == &smack_known_star || skp == &smack_known_web)))
>  			rc = -EINVAL;
> diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
> index 3c720ff..56a1439 100644
> --- a/security/smack/smackfs.c
> +++ b/security/smack/smackfs.c
> @@ -1677,7 +1677,7 @@ static ssize_t smk_write_onlycap(struct file *file, const char __user *buf,
>  	if (smack_onlycap != NULL && smack_onlycap != skp)
>  		return -EPERM;
>  
> -	data = kzalloc(count, GFP_KERNEL);
> +	data = kzalloc(count + 1, GFP_KERNEL);
>  	if (data == NULL)
>  		return -ENOMEM;
>  
> @@ -2228,7 +2228,7 @@ static ssize_t smk_write_syslog(struct file *file, const char __user *buf,
>  	if (!smack_privileged(CAP_MAC_ADMIN))
>  		return -EPERM;
>  

How about putting a comment here explaining that this way if len is 0 you
make sure to have '\0' to avoid parsing problems?

> -	data = kzalloc(count, GFP_KERNEL);
> +	data = kzalloc(count + 1, GFP_KERNEL);
>  	if (data == NULL)
>  		return -ENOMEM;
>