Re: [PATCH v12 01/11] x86: text_poke() may access uninitialized struct pages

[Randy Dunlap]

On 06/21/2018 02:25 PM, Pavel Tatashin wrote:
> It supposed to be safe to modify static branches after jump_label_init().
> But, because static key modifying code eventually calls text_poke() we
> may end up with accessing struct page that have not been initialized.
> 
> Here is how to quickly reproduce the problem. Insert code like this
> into init/main.c:
> 
> | +static DEFINE_STATIC_KEY_FALSE(__test);
> | asmlinkage __visible void __init start_kernel(void)
> | {
> |        char *command_line;
> |@@ -587,6 +609,10 @@ asmlinkage __visible void __init start_kernel(void)
> |        vfs_caches_init_early();
> |        sort_main_extable();
> |        trap_init();
> |+       {
> |+       static_branch_enable(&__test);
> |+       WARN_ON(!static_branch_likely(&__test));
> |+       }
> |        mm_init();
> 
> The following warnings show-up:
> WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:701 text_poke+0x20d/0x230
> Modules linked in:
> CPU: 0 PID: 0 Comm: swapper Not tainted 4.18.0-rc1_pt_t1 #30
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> 1.11.0-20171110_100015-anatol 04/01/2014
> RIP: 0010:text_poke+0x20d/0x230
> Code: 0f 0b 4c 89 e2 4c 89 ee 4c 89 f7 e8 7d 4b 9b 00 31 d2 31 f6 bf 86 02
> 00 00 48 8b 05 95 8e 24 01 e8 78 18 d8 00 e9 55 ff ff ff <0f> 0b e9 54 fe
> ff ff 48 8b 05 75 a8 38 01 e9 64 fe ff ff 48 8b 1d
> RSP: 0000:ffffffff94e03e30 EFLAGS: 00010046
> RAX: 0100000000000000 RBX: fffff7b2c011f300 RCX: ffffffff94fcccf4
> RDX: 0000000000000001 RSI: ffffffff94e03e77 RDI: ffffffff94fcccef
> RBP: ffffffff94fcccef R08: 00000000fffffe00 R09: 00000000000000a0
> R10: 0000000000000000 R11: 0000000000000040 R12: 0000000000000001
> R13: ffffffff94e03e77 R14: ffffffff94fcdcef R15: fffff7b2c0000000
> FS:  0000000000000000(0000) GS:ffff9adc87c00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffff9adc8499d000 CR3: 000000000460a001 CR4: 00000000000606b0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  ? start_kernel+0x23e/0x4c8
>  ? start_kernel+0x23f/0x4c8
>  ? text_poke_bp+0x50/0xda
>  ? arch_jump_label_transform+0x89/0xe0
>  ? __jump_label_update+0x78/0xb0
>  ? static_key_enable_cpuslocked+0x4d/0x80
>  ? static_key_enable+0x11/0x20
>  ? start_kernel+0x23e/0x4c8
>  ? secondary_startup_64+0xa5/0xb0
> ---[ end trace abdc99c031b8a90a ]---
> 
> If the code above is moved after mm_init(), no warning is shown, as struct
> pages are initialized during handover from memblock.
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/text-patching.h |  1 +
>  arch/x86/kernel/alternative.c        | 10 +++++++++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> index 2ecd34e2d46c..e85ff65c43c3 100644
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -37,5 +37,6 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>  extern void *text_poke(void *addr, const void *opcode, size_t len);
>  extern int poke_int3_handler(struct pt_regs *regs);
>  extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
> +extern int after_bootmem;
>  
>  #endif /* _ASM_X86_TEXT_PATCHING_H */
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index a481763a3776..0230dbc3c599 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -686,13 +686,21 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
>   *
>   * Note: Must be called under text_mutex.
>   */
> -void *text_poke(void *addr, const void *opcode, size_t len)
> +void __ref *text_poke(void *addr, const void *opcode, size_t len)
>  {
>  	unsigned long flags;
>  	char *vaddr;
>  	struct page *pages[2];
>  	int i;
>  
> +	/* While boot memory allocator is runnig we cannot use struct

coding style:
	/*
	 * While boot memory ....

But more importantly, does this patch need to be backported for stable?


> +	 * pages as they are not yet initialized. However, we also know
> +	 * that this is early in boot, and it is safe to fallback to
> +	 * text_poke_early.
> +	 */
> +	if (unlikely(!after_bootmem))
> +		return text_poke_early(addr, opcode, len);
> +
>  	if (!core_kernel_text((unsigned long)addr)) {
>  		pages[0] = vmalloc_to_page(addr);
>  		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
> 

thanks,
-- 
~Randy