[BUG] security_release_secctx seems broken

[Casey Schaufler]

On 9/18/2017 1:25 PM, Casey Schaufler wrote:
> On 9/16/2017 11:18 AM, Konstantin Khlebnikov wrote:
>> I've got this kmemleak splat

Do you have a convenient test case? I'd like to 
verify my patch with your case if it is reasonable
to do so.

>>
>> unreferenced object 0xffff880f687ff6a8 (size 32):
>>   comm "cp", pid 4279, jiffies 4295784487 (age 2866.296s)
>>   hex dump (first 32 bytes):
>>     01 00 00 02 02 00 08 00 00 00 00 00 00 00 00 00  ................
>>     00 00 00 00 00 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  .....kkkkkkkkkk.
>>   backtrace:
>>     [<ffffffff8192d77a>] kmemleak_alloc+0x4a/0xa0
>>     [<ffffffff8125b721>] __kmalloc_track_caller+0x171/0x280
>>     [<ffffffff812125c7>] krealloc+0xa7/0xc0
>>     [<ffffffff812abb2d>] vfs_getxattr_alloc+0xbd/0x110
>>     [<ffffffff813ba789>] cap_inode_getsecurity+0x79/0x200
>>     [<ffffffff813be4d2>] security_inode_getsecurity+0x52/0x80
>>     [<ffffffff812aad1a>] xattr_getsecurity+0x3a/0xa0
>>     [<ffffffff812aadf4>] vfs_getxattr+0x74/0xa0
>>     [<ffffffff812ab27e>] getxattr+0x9e/0x170
>>     [<ffffffff812abd2a>] SyS_fgetxattr+0x5a/0x90
>>     [<ffffffff8193a67b>] entry_SYSCALL_64_fastpath+0x1e/0xae
>>     [<ffffffffffffffff>] 0xffffffffffffffff
>>
>> allocated in  xattr_getsecurity() security_inode_getsecurity() -> cap_inode_getsecurity()
>> should be freed by security_release_secctx() but there is no release_secctx hook.
> There is a bug here, but the fix suggested is not correct.
> security_inode_getsecurity() provides the security attribute
> with a specified name. In the past there would be exactly one
> use for this call, which was to return the value of the
> attribute that happened to match the security "context" of
> the inode. Freeing the value with security_release_secctx()
> works coincidentally. Because security_inode_getsecurity() is
> called with the "true" value for the alloc parameter the correct
> fix is:
>
> 	- fix smack_inode_getsecurity() to honor the alloc flag
> 	- change the security_release_secctx() to kfree here.
>
> I can provide a patch in a day or so.
>
>> I don't know details about modern security models stack but it
>> seems security_release_secctx() have to know which layer owns
>> secdata to free it property: selinux just calls kfree,
>> capability_hooks should do the same, but other modules returns
>> non-freeable pointers. Currently security_release_secctx() calls
>> release_secctx for all models, this is obviously wrong.
>>
>
> .
>

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