Re: [PATCH] arm64: asm-bug: Add .align 2 to the end of __BUG_ENTRY
From: Li Zetao
Date: Mon May 20 2024 - 08:06:00 EST
Hi,
On 2024/5/20 18:33, Mark Rutland wrote:
On Fri, May 17, 2024 at 10:13:28PM +0800, Jiangfeng Xiao wrote:
I'm using the latest linux kernel mainline code,
with the default arm64 configuration:
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- defconfig,
and set CONFIG_EXPERT=y, CONFIG_DEBUG_BUGVERBOSE=n,
CONFIG_PANIC_ON_OOPS=y.
Loading the following kernel module will cause kernel panic.
The call stack is as follows:
root@(none):/# /root/insmod hello.ko
[ 6.035003] hello: loading out-of-tree module taints kernel.
[ 6.039129] ------------[ cut here ]------------
[ 6.039287] hello
[ 6.039704] Unexpected kernel BRK exception at EL1
[ 6.040059] Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
[ 6.040457] Modules linked in: hello(O+)
[ 6.041311] CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
[ 6.041755] Hardware name: linux,dummy-virt (DT)
[ 6.042238] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 6.042594] pc : buginit+0x18/0x1000 [hello]
[ 6.043601] lr : buginit+0x18/0x1000 [hello]
[ 6.043852] sp : ffff800080533ae0
[ 6.044121] x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
[ 6.044523] x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
[ 6.044961] x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
[ 6.045503] x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
[ 6.046124] x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
[ 6.046387] x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
[ 6.046829] x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
[ 6.047293] x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
[ 6.047739] x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
[ 6.047955] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
[ 6.048366] Call trace:
[ 6.048653] buginit+0x18/0x1000 [hello]
[ 6.048922] do_one_initcall+0x80/0x1c8
[ 6.049333] do_init_module+0x60/0x218
[ 6.049475] load_module+0x1ba4/0x1d70
[ 6.049755] __do_sys_init_module+0x198/0x1d0
[ 6.049959] __arm64_sys_init_module+0x1c/0x28
[ 6.050160] invoke_syscall+0x48/0x114
[ 6.050334] el0_svc_common.constprop.0+0x40/0xe0
[ 6.050468] do_el0_svc+0x1c/0x28
[ 6.050635] el0_svc+0x34/0xd8
[ 6.050852] el0t_64_sync_handler+0x120/0x12c
[ 6.051088] el0t_64_sync+0x190/0x194
[ 6.051433] Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
[ 6.052212] ---[ end trace 0000000000000000 ]---
[ 6.052473] Kernel panic - not syncing: BRK handler: Fatal exception
The kernel module source code is as follows:
```
static int __init buginit(void)
{
WARN(1, "hello\n");
return 0;
}
static void __exit bugexit(void)
{
}
module_init(buginit);
module_exit(bugexit);
MODULE_LICENSE("GPL");
```
When CONFIG_DEBUG_BUGVERBOSE macro is disabled,
the size of "__bug_table" section in hello.ko
is only 6 bytes instead of the expected 8 bytes.
As a result,
mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry) = 6 / 8 = 0
calculated in module_bug_finalize when the kernel loads ko is incorrect.
When running `WARN()`, the following backtrace is triggered:
module_find_bug() at lib/bug.c
find_bug() at lib/bug.c
__report_bug() at lib/bug.c
report_bug() at lib/bug.c
call_break_hook() at arch/arm64/kernel/debug-monitors.c
brk_handler() at arch/arm64/kernel/debug-monitors.c
It will return -EFAULT because hello.ko's mod->num_bugs is 0.
Finally, the kernel OOPS is triggered.
Add .align 2 to the end of __BUG_ENTRY
to make the object layout generated by the assembly code
consistent with that of the C struct bug_entry.
Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
Signed-off-by: Yuanbin Xie <xieyuanbin1@xxxxxxxxxx>
Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@xxxxxxxxxx>
---
arch/arm64/include/asm/asm-bug.h | 1 +
1 file changed, 1 insertion(+)
Ouch; sorry about this.
I reckon we should spell out the reason for this a bit more; would you
be happy with the commit message below?
| When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
| to bug_table entries, and as a result the last entry in a bug table will
| be ignored, potentially leading to an unexpected panic(). All prior
| entries in the table will be handled correctly.
|
| The arm64 ABI requires that struct fields of up to 8 bytes are
| naturally-aligned, with padding added within a struct such that struct
| are suitably aligned within arrays.
|
| When CONFIG_DEBUG_BUGVERPOSE=y, the layout of a bug_entry is:
|
| struct bug_entry {
| signed int bug_addr_disp; // 4 bytes
| signed int file_disp; // 4 bytes
| unsigned short line; // 2 bytes
| unsigned short flags; // 2 bytes
| }
|
| ... with 12 bytes total, requiring 4-byte alignment.
|
| When CONFIG_DEBUG_BUGVERBOSE=n, the layout of a bug_entry is:
|
| struct bug_entry {
| signed int bug_addr_disp; // 4 bytes
| unsigned short flags; // 2 bytes
| < implicit padding > // 2 bytes
| }
|
| ... with 8 bytes total, with 6 bytes of data and 2 bytes of trailing
| padding, requiring 4-byte alginment.
|
| When we create a bug_entry in assembly, we align the start of the entry
| to 4 bytes, which implicitly handles padding for any prior entries.
| However, we do not align the end of the entry, and so when
| CONFIG_DEBUG_BUGVERBOSE=n, the final entry lacks the trailing padding
| bytes.
|
| For the main kernel image this is not a problem as find_bug() doesn't
| depend on the trailing padding bytes when searching for entries:
|
| for (bug = __start___bug_table; bug < __stop___bug_table; ++bug)
| if (bugaddr == bug_addr(bug))
| return bug;
|
| However for modules, module_bug_finalize() depends on the trailing
| bytes when calculating the number of entries:
|
| mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry);
|
| ... and as the last bug_entry lacks the necessary padding bytes, this entry
| will not be counted, e.g. in the case of a single entry:
|
| sechdrs[i].sh_size == 6
| sizeof(struct bug_entry) == 8;
|
| sechdrs[i].sh_size / sizeof(struct bug_entry) == 0;
|
| Consequently module_find_bug() will miss the last bug_entry when it does:
|
| for (i = 0; i < mod->num_bugs; ++i, ++bug)
| if (bugaddr == bug_addr(bug))
| goto out;
|
| ... which can lead to a kenrel panic due to an unhandled bug.
|
| This can be demonstrated with the following module:
|
| static int __init buginit(void)
| {
| WARN(1, "hello\n");
| return 0;
| }
|
| static void __exit bugexit(void)
| {
| }
|
| module_init(buginit);
| module_exit(bugexit);
| MODULE_LICENSE("GPL");
|
| ... which will trigger a kernel panic when loaded:
|
| ------------[ cut here ]------------
| hello
| Unexpected kernel BRK exception at EL1
| Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
| Modules linked in: hello(O+)
| CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
| Hardware name: linux,dummy-virt (DT)
| pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
| pc : buginit+0x18/0x1000 [hello]
| lr : buginit+0x18/0x1000 [hello]
| sp : ffff800080533ae0
| x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
| x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
| x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
| x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
| x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
| x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
| x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
| x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
| x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
| x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
| Call trace:
| buginit+0x18/0x1000 [hello]
| do_one_initcall+0x80/0x1c8
| do_init_module+0x60/0x218
| load_module+0x1ba4/0x1d70
| __do_sys_init_module+0x198/0x1d0
| __arm64_sys_init_module+0x1c/0x28
| invoke_syscall+0x48/0x114
| el0_svc_common.constprop.0+0x40/0xe0
| do_el0_svc+0x1c/0x28
| el0_svc+0x34/0xd8
| el0t_64_sync_handler+0x120/0x12c
| el0t_64_sync+0x190/0x194
| Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
| ---[ end trace 0000000000000000 ]---
| Kernel panic - not syncing: BRK handler: Fatal exception
|
| Fix this by always aligning the end of a bug_entry to 4 bytes, which is
| correct regardless of CONFIG_DEBUG_BUGVERBOSE.
|
| Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
| Signed-off-by: Yuanbin Xie <xieyuanbin1@xxxxxxxxxx>
| Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@xxxxxxxxxx>
With that:
Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Mark.
diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
index c762038..6e73809 100644
--- a/arch/arm64/include/asm/asm-bug.h
+++ b/arch/arm64/include/asm/asm-bug.h
@@ -28,6 +28,7 @@
14470: .long 14471f - .; \
_BUGVERBOSE_LOCATION(__FILE__, __LINE__) \
.short flags; \
+ .align 2; \
The use of .align 2 here is based on the assumption that struct
bug_entry is 4-byte aligned. Currently, there is no problem with this
assumption, but for compatibility reasons, refer to the riscv
architecture and refactor the implementation of __BUG_FLAGS:
#define __BUG_FLAGS(flags) \
do { \
__asm__ __volatile__ ( \
"1:\n\t" \
"ebreak\n" \
".pushsection __bug_table,\"aw\"\n\t" \
"2:\n\t" \
__BUG_ENTRY "\n\t" \
".org 2b + %3\n\t" \
".popsection" \
: \
: "i" (__FILE__), "i" (__LINE__), \
"i" (flags), \
"i" (sizeof(struct bug_entry))); \
} while (0)
Align the real size of struct bug_entry through .org. What do you think?
.popsection; \
14471:
#else
--
1.8.5.6