[tip: x86/urgent] x86/entry: Assert that syscalls are on the right stack

[tip-bot2 for Andy Lutomirski]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     c9c26150e61de441ab58b25c1f64afc049ee0fee
Gitweb:        https://git.kernel.org/tip/c9c26150e61de441ab58b25c1f64afc049ee0fee
Author:        Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate:    Fri, 26 Jun 2020 10:21:11 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Wed, 01 Jul 2020 10:00:25 +02:00

x86/entry: Assert that syscalls are on the right stack

Now that the entry stack is a full page, it's too easy to regress the
system call entry code and end up on the wrong stack without noticing.
Assert that all system calls (SYSCALL64, SYSCALL32, SYSENTER, and INT80)
are on the right stack and have pt_regs in the right place.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/52059e42bb0ab8551153d012d68f7be18d72ff8e.1593191971.git.luto@xxxxxxxxxx

---
 arch/x86/entry/common.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index bd3f141..ed8ccc8 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -45,6 +45,15 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
 
+/* Check that the stack and regs on entry from user mode are sane. */
+static void check_user_regs(struct pt_regs *regs)
+{
+	if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) {
+		WARN_ON_ONCE(!on_thread_stack());
+		WARN_ON_ONCE(regs != task_pt_regs(current));
+	}
+}
+
 #ifdef CONFIG_CONTEXT_TRACKING
 /**
  * enter_from_user_mode - Establish state when coming from user mode
@@ -127,9 +136,6 @@ static long syscall_trace_enter(struct pt_regs *regs)
 	unsigned long ret = 0;
 	u32 work;
 
-	if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
-		BUG_ON(regs != task_pt_regs(current));
-
 	work = READ_ONCE(ti->flags);
 
 	if (work & (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU)) {
@@ -346,6 +352,8 @@ __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
 {
 	struct thread_info *ti;
 
+	check_user_regs(regs);
+
 	enter_from_user_mode();
 	instrumentation_begin();
 
@@ -409,6 +417,8 @@ static void do_syscall_32_irqs_on(struct pt_regs *regs)
 /* Handles int $0x80 */
 __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 {
+	check_user_regs(regs);
+
 	enter_from_user_mode();
 	instrumentation_begin();
 
@@ -460,6 +470,8 @@ __visible noinstr long do_fast_syscall_32(struct pt_regs *regs)
 					vdso_image_32.sym_int80_landing_pad;
 	bool success;
 
+	check_user_regs(regs);
+
 	/*
 	 * SYSENTER loses EIP, and even SYSCALL32 needs us to skip forward
 	 * so that 'regs->ip -= 2' lands back on an int $0x80 instruction.