Re: [PATCH] livepatch: Fix ftrace module text permissions race

From: Miroslav Benes
Date: Fri May 31 2019 - 04:52:47 EST


On Wed, 29 May 2019, Josh Poimboeuf wrote:

> It's possible for livepatch and ftrace to be toggling a module's text
> permissions at the same time, resulting in the following panic:
>
> BUG: unable to handle page fault for address: ffffffffc005b1d9
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0003) - permissions violation
> PGD 3ea0c067 P4D 3ea0c067 PUD 3ea0e067 PMD 3cc13067 PTE 3b8a1061
> Oops: 0003 [#1] PREEMPT SMP PTI
> CPU: 1 PID: 453 Comm: insmod Tainted: G O K 5.2.0-rc1-a188339ca5 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-20181126_142135-anatol 04/01/2014
> RIP: 0010:apply_relocate_add+0xbe/0x14c
> Code: fa 0b 74 21 48 83 fa 18 74 38 48 83 fa 0a 75 40 eb 08 48 83 38 00 74 33 eb 53 83 38 00 75 4e 89 08 89 c8 eb 0a 83 38 00 75 43 <89> 08 48 63 c1 48 39 c8 74 2e eb 48 83 38 00 75 32 48 29 c1 89 08
> RSP: 0018:ffffb223c00dbb10 EFLAGS: 00010246
> RAX: ffffffffc005b1d9 RBX: 0000000000000000 RCX: ffffffff8b200060
> RDX: 000000000000000b RSI: 0000004b0000000b RDI: ffff96bdfcd33000
> RBP: ffffb223c00dbb38 R08: ffffffffc005d040 R09: ffffffffc005c1f0
> R10: ffff96bdfcd33c40 R11: ffff96bdfcd33b80 R12: 0000000000000018
> R13: ffffffffc005c1f0 R14: ffffffffc005e708 R15: ffffffff8b2fbc74
> FS: 00007f5f447beba8(0000) GS:ffff96bdff900000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffffffffc005b1d9 CR3: 000000003cedc002 CR4: 0000000000360ea0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> klp_init_object_loaded+0x10f/0x219
> ? preempt_latency_start+0x21/0x57
> klp_enable_patch+0x662/0x809
> ? virt_to_head_page+0x3a/0x3c
> ? kfree+0x8c/0x126
> patch_init+0x2ed/0x1000 [livepatch_test02]
> ? 0xffffffffc0060000
> do_one_initcall+0x9f/0x1c5
> ? kmem_cache_alloc_trace+0xc4/0xd4
> ? do_init_module+0x27/0x210
> do_init_module+0x5f/0x210
> load_module+0x1c41/0x2290
> ? fsnotify_path+0x3b/0x42
> ? strstarts+0x2b/0x2b
> ? kernel_read+0x58/0x65
> __do_sys_finit_module+0x9f/0xc3
> ? __do_sys_finit_module+0x9f/0xc3
> __x64_sys_finit_module+0x1a/0x1c
> do_syscall_64+0x52/0x61
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> The above panic occurs when loading two modules at the same time with
> ftrace enabled, where at least one of the modules is a livepatch module:
>
> CPU0 CPU1
> klp_enable_patch()
> klp_init_object_loaded()
> module_disable_ro()
> ftrace_module_enable()
> ftrace_arch_code_modify_post_process()
> set_all_modules_text_ro()
> klp_write_object_relocations()
> apply_relocate_add()
> *patches read-only code* - BOOM
>
> A similar race exists when toggling ftrace while loading a livepatch
> module.
>
> Fix it by ensuring that the livepatch and ftrace code patching
> operations -- and their respective permissions changes -- are protected
> by the text_mutex.
>
> Reported-by: Johannes Erdfelt <johannes@xxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

For the code

Reviewed-by: Miroslav Benes <mbenes@xxxxxxx>

However, shouldn't the patch be split in two? One adding text_mutex
protection to livepatch and ftrace. The other adding lockdep_assert_held()
and __module_enable_ro()? The current changelog does not mention lockdep
changes at all.

Miroslav