linux-kernel - [PATCH] entry: remove empty addr_limit_user

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Date:   Mon, 21 Aug 2023 17:35:26 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org
Cc:     luto@...nel.org, mark.rutland@....com, peterz@...radead.org,
        tglx@...utronix.de
Subject: [PATCH] entry: remove empty addr_limit_user_check()

Back when set_fs() was a generic API for altering the address limit,
addr_limit_user_check() was a safety measure to prevent userspace being
able to issue syscalls with an unbound limit.

With the the removal of set_fs() as a generic API, the last user of
addr_limit_user_check() was removed in commit:

  b5a5a01d8e9a44ec ("arm64: uaccess: remove addr_limit_user_check()")

... as since that commit, no architecture defines TIF_FSCHECK, and hence
addr_limit_user_check() always expands to nothing.

Remove addr_limit_user_check(), updating the comment in
exit_to_user_mode_prepare() to no longer refer to it. At the same time,
the comment is reworded to be a little more generic so as to cover
kmap_assert_nomap() in addition to lockdep_sys_exit().

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org
---
 include/linux/syscalls.h | 16 ----------------
 kernel/entry/common.c    |  3 +--
 2 files changed, 1 insertion(+), 18 deletions(-)

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 03e3d0121d5e3..c4b9b66b0d053 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -283,22 +283,6 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
 #define SYSCALL32_DEFINE6 SYSCALL_DEFINE6
 #endif
 
-/*
- * Called before coming back to user-mode. Returning to user-mode with an
- * address limit different than USER_DS can allow to overwrite kernel memory.
- */
-static inline void addr_limit_user_check(void)
-{
-#ifdef TIF_FSCHECK
-	if (!test_thread_flag(TIF_FSCHECK))
-		return;
-#endif
-
-#ifdef TIF_FSCHECK
-	clear_thread_flag(TIF_FSCHECK);
-#endif
-}
-
 /*
  * These syscall function prototypes are kept in the same order as
  * include/uapi/asm-generic/unistd.h. Architecture specific entries go below,
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index be61332c66b54..d7ee4bc3f2ba3 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -205,8 +205,7 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
 
 	arch_exit_to_user_mode_prepare(regs, ti_work);
 
-	/* Ensure that the address limit is intact and no locks are held */
-	addr_limit_user_check();
+	/* Ensure that kernel state is sane for a return to userspace */
 	kmap_assert_nomap();
 	lockdep_assert_irqs_disabled();
 	lockdep_sys_exit();
-- 
2.30.2