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

Steve Best requested to merge sfbest/centos-stream-9:26672 into main

JIRA: https://issues.redhat.com/browse/RHEL-26672 CVE: CVE-2024-26603 Build Info: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=59366301 Tested: Did sanity fpu testing using tst-pkey from glibc-support.

commit d877550eaf2dc9090d782864c96939397a3c6835 Author: Andrei Vagin avagin@google.com Date: Mon Jan 29 22:36:03 2024 -0800

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 ]

Fixes: fcb3635f5018 ("x86/fpu/signal: Handle #PF in the direct restore path")
Reported-by: Konstantin Bogomolov <bogomolov@google.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240130063603.3392627-1-avagin%40google.com

Signed-off-by: Steve Best sbest@redhat.com

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