[PATCH v3 3/7] arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return

[Will Deacon <will@xxxxxxxxxx>]

Although we zero the upper bits of x0 on entry to the kernel from an
AArch32 task, we do not clear them on the exception return path and can
therefore expose 64-bit sign extended syscall return values to userspace
via interfaces such as the 'perf_regs' ABI, which deal exclusively with
64-bit registers.

Explicitly clear the upper 32 bits of x0 on return from a compat system
call.

Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Keno Fischer <keno@xxxxxxxxxxxxxxxxxx>
Cc: Luis Machado <luis.machado@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 arch/arm64/include/asm/syscall.h | 12 +++++++++++-
 arch/arm64/kernel/syscall.c      |  3 +++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index 65299a2dcf9c..cfc0672013f6 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -34,6 +34,10 @@ static inline long syscall_get_error(struct task_struct *task,
 				     struct pt_regs *regs)
 {
 	unsigned long error = regs->regs[0];
+
+	if (is_compat_thread(task_thread_info(task)))
+		error = sign_extend64(error, 31);
+
 	return IS_ERR_VALUE(error) ? error : 0;
 }
 
@@ -47,7 +51,13 @@ static inline void syscall_set_return_value(struct task_struct *task,
 					    struct pt_regs *regs,
 					    int error, long val)
 {
-	regs->regs[0] = (long) error ? error : val;
+	if (error)
+		val = error;
+
+	if (is_compat_thread(task_thread_info(task)))
+		val = lower_32_bits(val);
+
+	regs->regs[0] = val;
 }
 
 #define SYSCALL_MAX_ARGS 6
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 7c14466a12af..98a26d4e7b0c 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -50,6 +50,9 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 		ret = do_ni_syscall(regs, scno);
 	}
 
+	if (is_compat_task())
+		ret = lower_32_bits(ret);
+
 	regs->regs[0] = ret;
 }
 
-- 
2.27.0.383.g050319c2ae-goog