arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return

commit 15956689a0e60aa0c795174f3c310b60d8794235 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index 58102652bf9e38ed1b1b89545f2e4e1555d29f91..615376030abad98382506c7ae80bc59521b98dab 100644 (file)
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -45,6 +45,10 @@ 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;
 }
 
@@ -58,7 +62,13 @@ 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 b7b0d7f9ea05ac874a22ba1f7135bda48fb20458..1457a0ba83dbc7bbdbf8c7e5400fde4022f082df 100644 (file)
--- 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;
 }