arm64: fix compat syscall return truncation

Due to inconsistencies in the way we manipulate compat GPRs, we have a
few issues today:

* For audit and tracing, where error codes are handled as a (native)
  long, negative error codes are expected to be sign-extended to the
  native 64-bits, or they may fail to be matched correctly. Thus a
  syscall which fails with an error may erroneously be identified as
  failing.

* For ptrace, *all* compat return values should be sign-extended for
  consistency with 32-bit arm, but we currently only do this for
  negative return codes.

* As we may transiently set the upper 32 bits of some compat GPRs while
  in the kernel, these can be sampled by perf, which is somewhat
  confusing. This means that where a syscall returns a pointer above 2G,
  this will be sign-extended, but will not be mistaken for an error as
  error codes are constrained to the inclusive range [-4096, -1] where
  no user pointer can exist.

To fix all of these, we must consistently use helpers to get/set the
compat GPRs, ensuring that we never write the upper 32 bits of the
return code, and always sign-extend when reading the return code.  This
patch does so, with the following changes:

* We re-organise syscall_get_return_value() to always sign-extend for
  compat tasks, and reimplement syscall_get_error() atop. We update
  syscall_trace_exit() to use syscall_get_return_value().

* We consistently use syscall_set_return_value() to set the return
  value, ensureing the upper 32 bits are never set unexpectedly.

* As the core audit code currently uses regs_return_value() rather than
  syscall_get_return_value(), we special-case this for
  compat_user_mode(regs) such that this will do the right thing. Going
  forward, we should try to move the core audit code over to
  syscall_get_return_value().

Cc: <stable@vger.kernel.org>
Reported-by: He Zhe <zhe.he@windriver.com>
Reported-by: weiyuchen <weiyuchen3@huawei.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210802104200.21390-1-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 10 insertions, 9 deletions

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index cfc0672013f6..03e20895453a 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -29,22 +29,23 @@ static inline void syscall_rollback(struct task_struct *task,
 	regs->regs[0] = regs->orig_x0;
 }
 
-
-static inline long syscall_get_error(struct task_struct *task,
-				     struct pt_regs *regs)
+static inline long syscall_get_return_value(struct task_struct *task,
+					    struct pt_regs *regs)
 {
-	unsigned long error = regs->regs[0];
+	unsigned long val = regs->regs[0];
 
 	if (is_compat_thread(task_thread_info(task)))
-		error = sign_extend64(error, 31);
+		val = sign_extend64(val, 31);
 
-	return IS_ERR_VALUE(error) ? error : 0;
+	return val;
 }
 
-static inline long syscall_get_return_value(struct task_struct *task,
-					    struct pt_regs *regs)
+static inline long syscall_get_error(struct task_struct *task,
+				     struct pt_regs *regs)
 {
-	return regs->regs[0];
+	unsigned long error = syscall_get_return_value(task, regs);
+
+	return IS_ERR_VALUE(error) ? error : 0;
 }
 
 static inline void syscall_set_return_value(struct task_struct *task,