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Date: Tue, 9 Jul 2019 14:57:44 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Andy Lutomirski <luto@...nel.org>,
Ingo Molnar <mingo@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Linus Torvalds <torvalds@...ux-foundation.org>,
kernel test robot <rong.a.chen@...el.com>
Subject: Re: [PATCH] x86/alternatives: Fix int3_emulate_call() selftest stack
corruption
On Mon, Jul 08, 2019 at 03:55:30PM -0500, Josh Poimboeuf wrote:
> arch/x86/kernel/alternative.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 99ef8b6f9a1a..2644a7b82f96 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -651,6 +651,13 @@ int3_exception_notify(struct notifier_block *self, unsigned long val, void *data
> return NOTIFY_STOP;
> }
>
> +#ifdef CONFIG_X86_32
> +# define INT3_TEST_CLOBBERS "memory", "cc", "ecx", "edx"
> +#else
> +# define INT3_TEST_CLOBBERS "memory", "cc", "rax", "rcx", "rdx", "rsi", "r8", \
> + "r9", "r10", "r11"
> +#endif
> +
> static void __init int3_selftest(void)
> {
> static __initdata struct notifier_block int3_exception_nb = {
> @@ -676,7 +683,7 @@ static void __init int3_selftest(void)
> "int3_selftest_ip:\n\t"
> __ASM_SEL(.long, .quad) " 1b\n\t"
> ".popsection\n\t"
> - : : __ASM_SEL_RAW(a, D) (&val) : "memory");
> + : : __ASM_SEL_RAW(a, D) (&val) : INT3_TEST_CLOBBERS);
>
> BUG_ON(val != 1);
As discussed on IRC; I've changed that to the other variant.
---
Subject: x86/alternatives: Fix int3_emulate_call() selftest stack corruption
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon, 8 Jul 2019 15:55:30 -0500
KASAN shows the following splat during boot:
BUG: KASAN: unknown-crash in unwind_next_frame+0x3f6/0x490
Read of size 8 at addr ffffffff84007db0 by task swapper/0
CPU: 0 PID: 0 Comm: swapper Tainted: G T 5.2.0-rc6-00013-g7457c0d #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
Call Trace:
dump_stack+0x19/0x1b
print_address_description+0x1b0/0x2b2
? unwind_next_frame+0x3f6/0x490
__kasan_report+0x10f/0x171
? unwind_next_frame+0x3f6/0x490
kasan_report+0x12/0x1c
__asan_load8+0x54/0x81
unwind_next_frame+0x3f6/0x490
? unwind_dump+0x24e/0x24e
unwind_next_frame+0x1b/0x23
? create_prof_cpu_mask+0x20/0x20
arch_stack_walk+0x68/0xa5
? set_memory_4k+0x2a/0x2c
stack_trace_save+0x7b/0xa0
? stack_trace_consume_entry+0x89/0x89
save_trace+0x3c/0x93
mark_lock+0x1ef/0x9b1
? sched_clock_local+0x86/0xa6
__lock_acquire+0x3ba/0x1bea
? __asan_loadN+0xf/0x11
? mark_held_locks+0x8e/0x8e
? mark_lock+0xb4/0x9b1
? sched_clock_local+0x86/0xa6
lock_acquire+0x122/0x221
? _vm_unmap_aliases+0x141/0x183
__mutex_lock+0xb6/0x731
? _vm_unmap_aliases+0x141/0x183
? sched_clock_cpu+0xac/0xb1
? __mutex_add_waiter+0xae/0xae
? lock_downgrade+0x368/0x368
? _vm_unmap_aliases+0x40/0x183
mutex_lock_nested+0x16/0x18
_vm_unmap_aliases+0x141/0x183
? _vm_unmap_aliases+0x40/0x183
vm_unmap_aliases+0x14/0x16
change_page_attr_set_clr+0x15e/0x2f2
? __set_pages_p+0x111/0x111
? alternative_instructions+0xd8/0x118
? arch_init_ideal_nops+0x181/0x181
set_memory_4k+0x2a/0x2c
check_bugs+0x11fd/0x1298
? l1tf_cmdline+0x1dc/0x1dc
? proc_create_single_data+0x5f/0x6e
? cgroup_init+0x2b1/0x2f6
start_kernel+0x793/0x7eb
? thread_stack_cache_init+0x2e/0x2e
? idt_setup_early_handler+0x70/0xb1
x86_64_start_reservations+0x55/0x76
x86_64_start_kernel+0x87/0xaa
secondary_startup_64+0xa4/0xb0
Memory state around the buggy address:
ffffffff84007c80: 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1
ffffffff84007d00: f1 00 00 00 00 00 00 00 00 00 f2 f2 f2 f3 f3 f3
>ffffffff84007d80: f3 79 be 52 49 79 be 00 00 00 00 00 00 00 00 f1
It turns out that int3_selftest() is corrupting the stack. The problem
is that the KASAN-ified version of int3_magic() is much less trivial
than the C code appears. It clobbers several unexpected registers. So
when the selftest's INT3 is converted to an emulated call to
int3_magic(), the registers are clobbered and Bad Things happen when the
function returns.
Fix this by converting int3_magic() to the trivial ASM function it
should be, avoiding all calling convetion issues. Also add
ASM_CALL_CONSTRAINT to the INT3 ASM, since it contains a 'CALL'.
Fixes: 7457c0da024b ("x86/alternatives: Add int3_emulate_call() selftest")
Cc: x86@...nel.org
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Andy Lutomirski <luto@...nel.org>
Reported-by: kernel test robot <rong.a.chen@...el.com>
Debugged-by: Josh Poimboeuf <jpoimboe@...hat.com>
[peterz: cribbed changelog from josh]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/x86/kernel/alternative.c | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -625,10 +625,23 @@ extern struct paravirt_patch_site __star
*
* See entry_{32,64}.S for more details.
*/
-static void __init int3_magic(unsigned int *ptr)
-{
- *ptr = 1;
-}
+
+/*
+ * We define the int3_magic() function in assembly to control the calling
+ * convention such that we can 'call' it from assembly.
+ */
+
+extern void int3_magic(unsigned int *ptr); /* defined in asm */
+
+asm (
+" .pushsection .init.text, \"ax\", @progbits\n"
+" .type int3_magic, @function\n"
+"int3_magic:\n"
+" movl $1, (%" _ASM_ARG1 ")\n"
+" ret\n"
+" .size int3_magic, .-int3_magic\n"
+" .popsection\n"
+);
extern __initdata unsigned long int3_selftest_ip; /* defined in asm below */
@@ -676,7 +689,9 @@ static void __init int3_selftest(void)
"int3_selftest_ip:\n\t"
__ASM_SEL(.long, .quad) " 1b\n\t"
".popsection\n\t"
- : : __ASM_SEL_RAW(a, D) (&val) : "memory");
+ : ASM_CALL_CONSTRAINT
+ : __ASM_SEL_RAW(a, D) (&val)
+ : "memory");
BUG_ON(val != 1);
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