From: Will Deacon <will@kernel.org>
Date: Fri, 3 Jul 2020 12:08:42 +0100
Subject: arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return
Git-commit: 15956689a0e60aa0c795174f3c310b60d8794235
Patch-mainline: v5.8-rc6
References: git-fixes

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@vger.kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Keno Fischer <keno@juliacomputing.com>
Cc: Luis Machado <luis.machado@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>
Acked-by: Ivan T. Ivanov <iivanov@suse.de>
---
 arch/arm64/include/asm/syscall.h |   12 +++++++++++-
 arch/arm64/kernel/syscall.c      |    3 +++
 2 files changed, 14 insertions(+), 1 deletion(-)

--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -34,6 +34,10 @@ static inline long syscall_get_error(str
 				     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_va
 					    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
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -50,6 +50,9 @@ static void invoke_syscall(struct pt_reg
 		ret = do_ni_syscall(regs, scno);
 	}
 
+	if (is_compat_task())
+		ret = lower_32_bits(ret);
+
 	regs->regs[0] = ret;
 }