Re: [PATCH v10 1/3] x86/syscalls: Check address limit on user-mode return

On Tue, Jun 20, 2017 at 1:24 PM, Kees Cook [off-list ref] wrote:

On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier [off-list ref] wrote:

quoted

Ensure the address limit is a user-mode segment before returning to
user-mode. Otherwise a process can corrupt kernel-mode memory and elevate
privileges [1].

The set_fs function sets the TIF_SETFS flag to force a slow path on
return. In the slow path, the address limit is checked to be USER_DS if
needed.

The addr_limit_user_check function is added as a cross-architecture
function to check the address limit.

[1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990

Signed-off-by: Thomas Garnier <redacted>

Thanks for reworking this series!

The bad state correctly BUGs under the LKDTM test:

[   21.171586] lkdtm: Performing direct entry CORRUPT_USER_DS
[   21.172791] lkdtm: setting bad task size limit
[   21.173742] ------------[ cut here ]------------
[   21.174641] kernel BUG at ./include/linux/syscalls.h:220!
...
[   21.193166] Call Trace:
[   21.193617]  ? trace_hardirqs_on_thunk+0x1a/0x1c
[   21.194443]  entry_SYSCALL64_slow_path+0x25/0x25


Tested-by: Kees Cook <redacted>

Is everyone happy with this patch for x86? Does this need anything
more/different?

Thanks!

-Kees

-- 
Kees Cook
Pixel Security