[PATCH v2] x86/unwind/orc: fix inactive tasks with stack pointer in %sp

[Jiri Slaby]

gcc 10 optimizes the scheduler code differently than its predecessors.
When DEBUG_SECTION_MISMATCH config is enabled, Makefile forces gcc not
to inline some functions (-fno-inline-functions-called-once). Before gcc
10, "no-inlined" __schedule starts with the usual prologue (push %bp; mov
%sp,%bp). So ORC unwinder simply picks stack pointer from %bp and
unwinds from __schedule just perfectly:
$ cat /proc/1/stack
[<0>] ep_poll+0x3e9/0x450
[<0>] do_epoll_wait+0xaa/0xc0
[<0>] __x64_sys_epoll_wait+0x1a/0x20
[<0>] do_syscall_64+0x33/0x40
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

But now, with gcc 10, there is no %bp prologue in __schedule:
$ cat /proc/1/stack
<nothing>

The orc entry of the point in __schedule is:
sp:sp+88 bp:last_sp-48 type:call end:0

In this case, nobody subtracts sizeof "struct inactive_task_frame" in
__unwind_start. The struct is put on the stack by __switch_to_asm and
only then __switch_to_asm stores %sp to task->thread.sp. But we start
unwinding from a point in __schedule (stored in frame->ret_addr by
'call') and not in __switch_to_asm.

So for these example values in __unwind_start:
sp=ffff94b50001fdc8 bp=ffff8e1f41d29340 ip=__schedule+0x1f0

The stack is:
 ffff94b50001fdc8: ffff8e1f41578000 # struct inactive_task_frame
 ffff94b50001fdd0: 0000000000000000
 ffff94b50001fdd8: ffff8e1f41d29340
 ffff94b50001fde0: ffff8e1f41611d40 # ...
 ffff94b50001fde8: ffffffff93c41920 # bx
 ffff94b50001fdf0: ffff8e1f41d29340 # bp
 ffff94b50001fdf8: ffffffff9376cad0 # ret_addr (and end of the struct)

0xffffffff9376cad0 is __schedule+0x1f0 (after the call to
__switch_to_asm).  Now follow those 88 bytes from the ORC entry (sp+88).
The entry is correct, __schedule really pushes 48 bytes (8*7) + 32 bytes
via subq to store some local values (like 4U below). So to unwind, look
at the offset 88-sizeof(long) = 0x50 from here:

 ffff94b50001fe00: ffff8e1f41578618
 ffff94b50001fe08: 00000cc000000255
 ffff94b50001fe10: 0000000500000004
 ffff94b50001fe18: 7793fab6956b2d00 # NOTE (see below)
 ffff94b50001fe20: ffff8e1f41578000
 ffff94b50001fe28: ffff8e1f41578000
 ffff94b50001fe30: ffff8e1f41578000
 ffff94b50001fe38: ffff8e1f41578000
 ffff94b50001fe40: ffff94b50001fed8
 ffff94b50001fe48: ffff8e1f41577ff0
 ffff94b50001fe50: ffffffff9376cf12

Here               ^^^^^^^^^^^^^^^^ is the correct ret addr from
__schedule. It translates to schedule+0x42 (insn after a call to
__schedule).

BUT, unwind_next_frame tries to take the address starting from
0xffff94b50001fdc8. That is exactly from thread.sp+88-sizeof(long) =
0xffff94b50001fdc8+88-8 = 0xffff94b50001fe18, which is garbage marked as
NOTE above. So this quits the unwinding as 7793fab6956b2d00 is obviously
not a kernel address.

There was a fix to skip 'struct inactive_task_frame' in
unwind_get_return_address_ptr in commit 187b96db5ca7 ("x86/unwind/orc:
Fix unwind_get_return_address_ptr() for inactive tasks").

But we need to skip the struct already in the unwinder proper. So
subtract the size (increase the stack pointer) of the structure in
__unwind_start directly. This allows for removal of the code added by
commit 187b96db5ca7 completely, as the address is now at
'(unsigned long *)state->sp - 1', the same as in the generic case.

Fixes: ee9f8fce9964 ("x86/unwind: Add the ORC unwinder")
Bug: https://bugzilla.suse.com/show_bug.cgi?id=1176907
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Cc: Miroslav Benes <mbenes@xxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: live-patching@xxxxxxxxxxxxxxx
---

[v2]
  * Remove comment from __unwind_start.
  * Cc more parties
  * Polish the commitlog

 arch/x86/kernel/unwind_orc.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index 6a339ce328e0..73f800100066 100644
--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -321,19 +321,12 @@ EXPORT_SYMBOL_GPL(unwind_get_return_address);
 
 unsigned long *unwind_get_return_address_ptr(struct unwind_state *state)
 {
-	struct task_struct *task = state->task;
-
 	if (unwind_done(state))
 		return NULL;
 
 	if (state->regs)
 		return &state->regs->ip;
 
-	if (task != current && state->sp == task->thread.sp) {
-		struct inactive_task_frame *frame = (void *)task->thread.sp;
-		return &frame->ret_addr;
-	}
-
 	if (state->sp)
 		return (unsigned long *)state->sp - 1;
 
@@ -663,7 +656,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	} else {
 		struct inactive_task_frame *frame = (void *)task->thread.sp;
 
-		state->sp = task->thread.sp;
+		state->sp = task->thread.sp + sizeof(*frame);
 		state->bp = READ_ONCE_NOCHECK(frame->bp);
 		state->ip = READ_ONCE_NOCHECK(frame->ret_addr);
 		state->signal = (void *)state->ip == ret_from_fork;
-- 
2.28.0