Subject: CVE-2024-26603: x86/fpu: Stop relying on userspace for info to fault in xsave buffer

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

x86/fpu: Stop relying on userspace for info to fault in xsave buffer

Before this change, the expected size of the user space buffer was
taken from fx_sw->xstate_size. fx_sw->xstate_size can be changed
from user-space, so it is possible construct a sigreturn frame where:

 * fx_sw->xstate_size is smaller than the size required by valid bits in
   fx_sw->xfeatures.
 * user-space unmaps parts of the sigrame fpu buffer so that not all of
   the buffer required by xrstor is accessible.

In this case, xrstor tries to restore and accesses the unmapped area
which results in a fault. But fault_in_readable succeeds because buf +
fx_sw->xstate_size is within the still mapped area, so it goes back and
tries xrstor again. It will spin in this loop forever.

Instead, fault in the maximum size which can be touched by XRSTOR (taken
from fpstate->user_size).

[ dhansen: tweak subject / changelog ]

The Linux kernel CVE team has assigned CVE-2024-26603 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.14 with commit fcb3635f5018 and fixed in 5.15.150 with commit 8bd3eee7720c
	Issue introduced in 5.14 with commit fcb3635f5018 and fixed in 6.1.79 with commit 627339cccdc9
	Issue introduced in 5.14 with commit fcb3635f5018 and fixed in 6.6.18 with commit b2479ab426ce
	Issue introduced in 5.14 with commit fcb3635f5018 and fixed in 6.7.6 with commit 627e28cbb655
	Issue introduced in 5.14 with commit fcb3635f5018 and fixed in 6.8 with commit d877550eaf2d

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26603
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/x86/kernel/fpu/signal.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/8bd3eee7720c14b59a206bd05b98d7586bccf99a
	https://git.kernel.org/stable/c/627339cccdc9166792ecf96bc3c9f711a60ce996
	https://git.kernel.org/stable/c/b2479ab426cef7ab79a13005650eff956223ced2
	https://git.kernel.org/stable/c/627e28cbb65564e55008315d9e02fbb90478beda
	https://git.kernel.org/stable/c/d877550eaf2dc9090d782864c96939397a3c6835