Commit 6fffacb3 authored Jul 19, 2018 by Pavel Tatashin Committed by Thomas Gleixner Jul 20, 2018

x86/alternatives, jumplabel: Use text_poke_early() before mm_init()



It supposed to be safe to modify static branches after jump_label_init().
But, because static key modifying code eventually calls text_poke() it can
end up accessing a struct page which has not been initialized yet.

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
RIP: 0010:text_poke+0x20d/0x230
Call Trace:
 ? 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.

Use text_poke_early() in static branching until early boot IRQs are enabled
and from there switch to text_poke. Also, ensure text_poke() is never
invoked when unitialized memory access may happen by using adding a
!after_bootmem assertion.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: steven.sistare@oracle.com
Cc: daniel.m.jordan@oracle.com
Cc: linux@armlinux.org.uk
Cc: schwidefsky@de.ibm.com
Cc: heiko.carstens@de.ibm.com
Cc: john.stultz@linaro.org
Cc: sboyd@codeaurora.org
Cc: hpa@zytor.com
Cc: douly.fnst@cn.fujitsu.com
Cc: peterz@infradead.org
Cc: prarit@redhat.com
Cc: feng.tang@intel.com
Cc: pmladek@suse.com
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: linux-s390@vger.kernel.org
Cc: boris.ostrovsky@oracle.com
Cc: jgross@suse.com
Cc: pbonzini@redhat.com
Link: https://lkml.kernel.org/r/20180719205545.16512-9-pasha.tatashin@oracle.com

parent 95a3d445

arch/x86/include/asm/text-patching.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -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 */

arch/x86/kernel/alternative.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -668,6 +668,7 @@ void __init_or_module text_poke_early(void addr, const void *opcode,
		local_irq_save(flags);
		memcpy(addr, opcode, len);
		local_irq_restore(flags);
		sync_core();
		/* Could also do a CLFLUSH here to speed up CPU recovery; but
		that causes hangs on some VIA CPUs. */
		return addr;
		@@ -693,6 +694,12 @@ void text_poke(void addr, const void *opcode, size_t len)
		struct page *pages[2];
		int i;

		/*
		* While boot memory allocator is runnig we cannot use struct
		* pages as they are not yet initialized.
		*/
		BUG_ON(!after_bootmem);

		if (!core_kernel_text((unsigned long)addr)) {
		pages[0] = vmalloc_to_page(addr);
		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);

arch/x86/kernel/jump_label.c

+7 −4

Original line number	Diff line number	Diff line
		@@ -37,7 +37,7 @@ static void bug_at(unsigned char *ip, int line)
		BUG();
		}

		static void __jump_label_transform(struct jump_entry *entry,
		static void __ref __jump_label_transform(struct jump_entry *entry,
		enum jump_label_type type,
		void (poker)(void , const void , size_t),
		int init)
		@@ -46,6 +46,9 @@ static void __jump_label_transform(struct jump_entry *entry,
		const unsigned char default_nop[] = { STATIC_KEY_INIT_NOP };
		const unsigned char *ideal_nop = ideal_nops[NOP_ATOMIC5];

		if (early_boot_irqs_disabled)
		poker = text_poke_early;

		if (type == JUMP_LABEL_JMP) {
		if (init) {
		/*