[PATCH 3.16 074/192] arm64: ptrace: Avoid setting compat FP[SC]R to garbage if get_user fails

[Ben Hutchings]

3.16.49-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Dave Martin <Dave.Martin@xxxxxxx>

commit 53b1a742ed251780267a57415bc955bd50f40c3d upstream.

If get_user() fails when reading the new FPSCR value from userspace
in compat_vfp_get(), then garbage* will be written to the task's
FPSR and FPCR registers.

This patch prevents this by checking the return from get_user()
first.

[*] Actually, zero, due to the behaviour of get_user() on error, but
that's still not what userspace expects.

Fixes: 478fcb2cdb23 ("arm64: Debugging support")
Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/arm64/kernel/ptrace.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -787,8 +787,10 @@ static int compat_vfp_set(struct task_st
 
 	if (count && !ret) {
 		ret = get_user(fpscr, (compat_ulong_t *)ubuf);
-		uregs->fpsr = fpscr & VFP_FPSCR_STAT_MASK;
-		uregs->fpcr = fpscr & VFP_FPSCR_CTRL_MASK;
+		if (!ret) {
+			uregs->fpsr = fpscr & VFP_FPSCR_STAT_MASK;
+			uregs->fpcr = fpscr & VFP_FPSCR_CTRL_MASK;
+		}
 	}
 
 	fpsimd_flush_task_state(target);