Re: [PATCH] selinux: KASAN: slab-out-of-bounds in xattr_getsecurity

[Stephen Smalley]

On 05/30/2018 11:19 AM, Paul Moore wrote:
> On Fri, May 25, 2018 at 4:31 AM, Sachin Grover <sgrover@xxxxxxxxxxxxxx> wrote:
>> Call trace:
>>  [<ffffff9203a8d7a8>] dump_backtrace+0x0/0x428
>>  [<ffffff9203a8dbf8>] show_stack+0x28/0x38
>>  [<ffffff920409bfb8>] dump_stack+0xd4/0x124
>>  [<ffffff9203d187e8>] print_address_description+0x68/0x258
>>  [<ffffff9203d18c00>] kasan_report.part.2+0x228/0x2f0
>>  [<ffffff9203d1927c>] kasan_report+0x5c/0x70
>>  [<ffffff9203d1776c>] check_memory_region+0x12c/0x1c0
>>  [<ffffff9203d17cdc>] memcpy+0x34/0x68
>>  [<ffffff9203d75348>] xattr_getsecurity+0xe0/0x160
>>  [<ffffff9203d75490>] vfs_getxattr+0xc8/0x120
>>  [<ffffff9203d75d68>] getxattr+0x100/0x2c8
>>  [<ffffff9203d76fb4>] SyS_fgetxattr+0x64/0xa0
>>  [<ffffff9203a83f70>] el0_svc_naked+0x24/0x28
>>
>> If user get root access and calls security.selinux setxattr() with an
>> embedded NUL on a file and then if some process performs a getxattr()
>> on that file with a length greater than the actual length of the string,
>> it would result in a panic.
>>
>> To fix this, add the actual length of the string to the security context
>> instead of the length passed by the userspace process.
>>
>> Signed-off-by: Sachin Grover <sgrover@xxxxxxxxxxxxxx>
>> ---
>>  security/selinux/ss/services.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Thanks for reporting this and providing a patch.  It's small enough,
> and passes all the regular tests, so I've merged it into
> selinux/stable-4.17 (adding the stable metadata) and I'm going to send
> it up to Linus today.
> 
> If Linus doesn't pull the fix in time for v4.17 I'll send it up during
> the upcoming merge window.

NB Such a setxattr() call can only be performed by a process with CAP_MAC_ADMIN that is also allowed mac_admin permission in SELinux policy. Consequently, this is never possible on Android (no process is allowed mac_admin permission, always enforcing) and is only possible in Fedora/RHEL for a few domains (if enforcing).

Fixes: 9a59daa03df72526d234b91dd3e32ded5aebd3ef ("SELinux: fix sleeping allocation in security_context_to_sid")

> 
>> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
>> index 66ea81c..d17f5b4 100644
>> --- a/security/selinux/ss/services.c
>> +++ b/security/selinux/ss/services.c
>> @@ -1434,7 +1434,7 @@ static int security_context_to_sid_core(const char *scontext, u32 scontext_len,
>>                                       scontext_len, &context, def_sid);
>>         if (rc == -EINVAL && force) {
>>                 context.str = str;
>> -               context.len = scontext_len;
>> +               context.len = strlen(str) + 1;
>>                 str = NULL;
>>         } else if (rc)
>>                 goto out_unlock;
>> --
>> 1.9.1
>