CVE-2021-47227

In the Linux kernel, the following vulnerability has been resolved:
x86/fpu: Prevent state corruption in __fpu__restore_sig() The non-compacted
slowpath uses __copy_from_user() and copies the entire user buffer into the
kernel buffer, verbatim. This means that the kernel buffer may now contain
entirely invalid state on which XRSTOR will #GP.
validate_user_xstate_header() can detect some of that corruption, but that
leaves the onus on callers to clear the buffer. Prior to XSAVES support, it
was possible just to reinitialize the buffer, completely, but with
supervisor states that is not longer possible as the buffer clearing code
split got it backwards. Fixing that is possible but not corrupting the
state in the first place is more robust. Avoid corruption of the kernel
XSAVE buffer by using copy_user_to_xstate() which validates the XSAVE
header contents before copying the actual states to the kernel.
copy_user_to_xstate() was previously only called for compacted-format
kernel buffers, but it works for both compacted and non-compacted forms.
Using it for the non-compacted form is slower because of multiple
__copy_from_user() operations, but that cost is less important than robust
code in an already slow path. [ Changelog polished by Dave Hansen ]