This is a note to let you know that I've just added the patch titled
mips: Call lose_fpu(0) before initializing fcr31 in mips_set_personality_nan
to the 4.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mips-call-lose_fpu-0-before-initializing-fcr31-in-mips_set_personality_nan.patch
and it can be found in the queue-4.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 59be5c35850171e307ca5d3d703ee9ff4096b948 Mon Sep 17 00:00:00 2001
From: Xi Ruoyao <xry111@xxxxxxxxxxx>
Date: Sat, 27 Jan 2024 05:05:57 +0800
Subject: mips: Call lose_fpu(0) before initializing fcr31 in mips_set_personality_nan
From: Xi Ruoyao <xry111@xxxxxxxxxxx>
commit 59be5c35850171e307ca5d3d703ee9ff4096b948 upstream.
If we still own the FPU after initializing fcr31, when we are preempted
the dirty value in the FPU will be read out and stored into fcr31,
clobbering our setting. This can cause an improper floating-point
environment after execve(). For example:
zsh% cat measure.c
#include <fenv.h>
int main() { return fetestexcept(FE_INEXACT); }
zsh% cc measure.c -o measure -lm
zsh% echo $((1.0/3)) # raising FE_INEXACT
0.33333333333333331
zsh% while ./measure; do ; done
(stopped in seconds)
Call lose_fpu(0) before setting fcr31 to prevent this.
Closes: https://lore.kernel.org/linux-mips/7a6aa1bbdbbe2e63ae96ff163fab0349f58f1b9e.camel@xxxxxxxxxxx/
Fixes: 9b26616c8d9d ("MIPS: Respect the ISA level in FCSR handling")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
Signed-off-by: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/mips/kernel/elf.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/arch/mips/kernel/elf.c
+++ b/arch/mips/kernel/elf.c
@@ -15,6 +15,7 @@
#include <asm/cpu-features.h>
#include <asm/cpu-info.h>
+#include <asm/fpu.h>
/* Whether to accept legacy-NaN and 2008-NaN user binaries. */
bool mips_use_nan_legacy;
@@ -311,6 +312,11 @@ void mips_set_personality_nan(struct arc
struct cpuinfo_mips *c = &boot_cpu_data;
struct task_struct *t = current;
+ /* Do this early so t->thread.fpu.fcr31 won't be clobbered in case
+ * we are preempted before the lose_fpu(0) in start_thread.
+ */
+ lose_fpu(0);
+
t->thread.fpu.fcr31 = c->fpu_csr31;
switch (state->nan_2008) {
case 0:
Patches currently in stable-queue which might be from xry111@xxxxxxxxxxx are
queue-4.19/mips-call-lose_fpu-0-before-initializing-fcr31-in-mips_set_personality_nan.patch
