Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 20 May 2024 11:33:37 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH] arm64: asm-bug: Add .align 2 to the end of __BUG_ENTRY |
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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@huawei.com> > Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@huawei.com> > --- > 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@huawei.com> | Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@huawei.com>
With that:
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
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; \ > .popsection; \ > 14471: > #else > -- > 1.8.5.6 > >
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