4.19-stable review patch. If anyone has any objections, please let me know.
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From: Xi Ruoyao xry111@xry111.site
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.... Fixes: 9b26616c8d9d ("MIPS: Respect the ISA level in FCSR handling") Cc: stable@vger.kernel.org Signed-off-by: Xi Ruoyao xry111@xry111.site Signed-off-by: Thomas Bogendoerfer tsbogend@alpha.franken.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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: