[tip: core/urgent] core/entry: Report syscall correctly for trace and audit

[tip-bot2 for Kees Cook]

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

Commit-ID:     b6ec413461034d49f9e586845825adb35ba308f6
Gitweb:        https://git.kernel.org/tip/b6ec413461034d49f9e586845825adb35ba308f6
Author:        Kees Cook <keescook@xxxxxxxxxxxx>
AuthorDate:    Fri, 11 Sep 2020 17:58:26 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Mon, 14 Sep 2020 22:49:51 +02:00

core/entry: Report syscall correctly for trace and audit

On v5.8 when doing seccomp syscall rewrites (e.g. getpid into getppid
as seen in the seccomp selftests), trace (and audit) correctly see the
rewritten syscall on entry and exit:

	seccomp_bpf-1307  [000] .... 22974.874393: sys_enter: NR 110 (...
	seccomp_bpf-1307  [000] .N.. 22974.874401: sys_exit: NR 110 = 1304

With mainline we see a mismatched enter and exit (the original syscall
is incorrectly visible on entry):

	seccomp_bpf-1030  [000] ....    21.806766: sys_enter: NR 39 (...
	seccomp_bpf-1030  [000] ....    21.806767: sys_exit: NR 110 = 1027

When ptrace or seccomp change the syscall, this needs to be visible to
trace and audit at that time as well. Update the syscall earlier so they
see the correct value.

Fixes: d88d59b64ca3 ("core/entry: Respect syscall number rewrites")
Reported-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200912005826.586171-1-keescook@xxxxxxxxxxxx

---
 kernel/entry/common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 1868359..6fdb610 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -60,13 +60,15 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
 			return ret;
 	}
 
+	/* Either of the above might have changed the syscall number */
+	syscall = syscall_get_nr(current, regs);
+
 	if (unlikely(ti_work & _TIF_SYSCALL_TRACEPOINT))
 		trace_sys_enter(regs, syscall);
 
 	syscall_enter_audit(regs, syscall);
 
-	/* The above might have changed the syscall number */
-	return ret ? : syscall_get_nr(current, regs);
+	return ret ? : syscall;
 }
 
 static __always_inline long