From: Will Deacon will@kernel.org
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
--- 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; }