Commit e836673c authored by Nadav Amit's avatar Nadav Amit Committed by Ingo Molnar
Browse files

x86/alternatives: Add text_poke_kgdb() to not assert the lock when debugging 



text_mutex is currently expected to be held before text_poke() is
called, but kgdb does not take the mutex, and instead *supposedly*
ensures the lock is not taken and will not be acquired by any other core
while text_poke() is running.

The reason for the "supposedly" comment is that it is not entirely clear
that this would be the case if gdb_do_roundup is zero.

Create two wrapper functions, text_poke() and text_poke_kgdb(), which do
or do not run the lockdep assertion respectively.

While we are at it, change the return code of text_poke() to something
meaningful. One day, callers might actually respect it and the existing
BUG_ON() when patching fails could be removed. For kgdb, the return
value can actually be used.

Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarNadav Amit <namit@vmware.com>
Signed-off-by: default avatarRick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
Acked-by: default avatarJiri Kosina <jkosina@suse.cz>
Cc: <akpm@linux-foundation.org>
Cc: <ard.biesheuvel@linaro.org>
Cc: <deneen.t.dock@intel.com>
Cc: <kernel-hardening@lists.openwall.com>
Cc: <kristen@linux.intel.com>
Cc: <linux_dti@icloud.com>
Cc: <will.deacon@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 9222f606 ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")
Link: https://lkml.kernel.org/r/20190426001143.4983-2-namit@vmware.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent d5963d87

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

+1 −0
Original line number Diff line number Diff line
@@ -35,6 +35,7 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len); 
 * inconsistent instruction while you patch.

 */

extern void *text_poke(void *addr, const void *opcode, size_t len);

extern void *text_poke_kgdb(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;

arch/x86/kernel/alternative.c

+38 −14
Original line number Diff line number Diff line
@@ -679,18 +679,7 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode, 
	return addr;

}



/**

 * text_poke - Update instructions on a live kernel

 * @addr: address to modify

 * @opcode: source of the copy

 * @len: length to copy

 *

 * Only atomic text poke/set should be allowed when not doing early patching.

 * It means the size must be writable atomically and the address must be aligned

 * in a way that permits an atomic write. It also makes sure we fit on a single

 * page.

 */

void *text_poke(void *addr, const void *opcode, size_t len)

static void *__text_poke(void *addr, const void *opcode, size_t len)

{

	unsigned long flags;

	char *vaddr;
@@ -703,8 +692,6 @@ void *text_poke(void *addr, const void *opcode, size_t len) 
	 */

	BUG_ON(!after_bootmem);



	lockdep_assert_held(&text_mutex);



	if (!core_kernel_text((unsigned long)addr)) {

		pages[0] = vmalloc_to_page(addr);

		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
@@ -733,6 +720,43 @@ void *text_poke(void *addr, const void *opcode, size_t len) 
	return addr;

}



/**

 * text_poke - Update instructions on a live kernel

 * @addr: address to modify

 * @opcode: source of the copy

 * @len: length to copy

 *

 * Only atomic text poke/set should be allowed when not doing early patching.

 * It means the size must be writable atomically and the address must be aligned

 * in a way that permits an atomic write. It also makes sure we fit on a single

 * page.

 */

void *text_poke(void *addr, const void *opcode, size_t len)

{

	lockdep_assert_held(&text_mutex);



	return __text_poke(addr, opcode, len);

}



/**

 * text_poke_kgdb - Update instructions on a live kernel by kgdb

 * @addr: address to modify

 * @opcode: source of the copy

 * @len: length to copy

 *

 * Only atomic text poke/set should be allowed when not doing early patching.

 * It means the size must be writable atomically and the address must be aligned

 * in a way that permits an atomic write. It also makes sure we fit on a single

 * page.

 *

 * Context: should only be used by kgdb, which ensures no other core is running,

 *	    despite the fact it does not hold the text_mutex.

 */

void *text_poke_kgdb(void *addr, const void *opcode, size_t len)

{

	return __text_poke(addr, opcode, len);

}



static void do_sync_core(void *info)

{

	sync_core();

arch/x86/kernel/kgdb.c

+6 −5
Original line number Diff line number Diff line
@@ -759,12 +759,12 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt) 
	if (!err)

		return err;

	/*

	 * It is safe to call text_poke() because normal kernel execution

	 * It is safe to call text_poke_kgdb() because normal kernel execution

	 * is stopped on all cores, so long as the text_mutex is not locked.

	 */

	if (mutex_is_locked(&text_mutex))

		return -EBUSY;

	text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,

	text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,

		       BREAK_INSTR_SIZE);

	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);

	if (err)
@@ -784,12 +784,13 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt) 
	if (bpt->type != BP_POKE_BREAKPOINT)

		goto knl_write;

	/*

	 * It is safe to call text_poke() because normal kernel execution

	 * It is safe to call text_poke_kgdb() because normal kernel execution

	 * is stopped on all cores, so long as the text_mutex is not locked.

	 */

	if (mutex_is_locked(&text_mutex))

		goto knl_write;

	text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE);

	text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,

		       BREAK_INSTR_SIZE);

	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);

	if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))

		goto knl_write;

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