On Thu, Sep 10, 2020 at 12:22:53PM -0500, Josh Poimboeuf wrote:
> The x86 uaccess code uses barrier_nospec() in various places to prevent
> speculative dereferencing of user-controlled pointers (which might be
> combined with further gadgets or CPU bugs to leak data).
>
> There are some issues with the current implementation:
>
> - The barrier_nospec() in copy_from_user() was inadvertently removed
> with: 4b842e4e25b1 ("x86: get rid of small constant size cases in
> raw_copy_{to,from}_user()")
>
> - copy_to_user() and friends should also have a speculation barrier,
> because a speculative write to a user-controlled address can still
> populate the cache line with the original data.
>
> - The LFENCE in barrier_nospec() is overkill, when more lightweight user
> pointer masking can be used instead.
>
> Remove all existing barrier_nospec() usage, and instead do user pointer
> masking, throughout the x86 uaccess code. This is similar to what arm64
> is already doing with uaccess_mask_ptr().
>
> barrier_nospec() is now unused, and can be removed.
>
> Fixes: 4b842e4e25b1 ("x86: get rid of small constant size cases in
> raw_copy_{to,from}_user()")
> Suggested-by: Will Deacon <[email protected]>
> Signed-off-by: Josh Poimboeuf <[email protected]>
> ---
> v3:
>
> - Rebased on vfs#for-next, using TASK_SIZE_MAX now that set_fs() is
> gone. I considered just clearing the most significant bit, but that
> only works for 64-bit, so in the interest of common code I went with
> the more straightforward enforcement of the TASK_SIZE_MAX limit.
>
> - Rename the macro to force_user_ptr(), which is more descriptive, and
> also more distinguishable from a planned future macro for sanitizing
> __user pointers on syscall entry.
>
> Documentation/admin-guide/hw-vuln/spectre.rst | 6 ++--
> arch/x86/include/asm/barrier.h | 3 --
> arch/x86/include/asm/checksum_32.h | 6 ++--
> arch/x86/include/asm/futex.h | 5 +++
> arch/x86/include/asm/uaccess.h | 35 ++++++++++++-------
> arch/x86/include/asm/uaccess_64.h | 16 ++++-----
> arch/x86/lib/csum-wrappers_64.c | 5 +--
> arch/x86/lib/getuser.S | 10 +++---
> arch/x86/lib/putuser.S | 8 +++++
> arch/x86/lib/usercopy_32.c | 6 ++--
> arch/x86/lib/usercopy_64.c | 7 ++--
> lib/iov_iter.c | 2 +-
> 12 files changed, 65 insertions(+), 44 deletions(-)
After clarifying some stuff on IRC:
Acked-by: Borislav Petkov <[email protected]>
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
