x86/fpu: Prevent state corruption in __fpu__restore_sig()

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 ]