6.6-stable review patch. If anyone has any objections, please let me know.
------------------
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
@@ -11,6 +11,7 @@
#include <asm/cpu-features.h>
#include <asm/cpu-info.h>
+#include <asm/fpu.h>
#ifdef CONFIG_MIPS_FP_SUPPORT
@@ -309,6 +310,11 @@ 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:
