Commit 35f1c89b authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Borislav Petkov
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x86/fault: Fix AMD erratum #91 errata fixup for user code 



The recent rework of probe_kernel_address() and its conversion to
get_kernel_nofault() inadvertently broke is_prefetch(). Before this
change, probe_kernel_address() was used as a sloppy "read user or
kernel memory" helper, but it doesn't do that any more. The new
get_kernel_nofault() reads *kernel* memory only, which completely broke
is_prefetch() for user access.

Adjust the code to the correct accessor based on access mode. The
manual address bounds check is no longer necessary, since the accessor
helpers (get_user() / get_kernel_nofault()) do the right thing all by
themselves. As a bonus, by using the correct accessor, the open-coded
address bounds check is not needed anymore.

 [ bp: Massage commit message. ]

Fixes: eab0c608 ("maccess: unify the probe kernel arch hooks")
Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/b91f7f92f3367d2d3a88eec3b09c6aab1b2dc8ef.1612924255.git.luto@kernel.org
parent 167dcfc0

arch/x86/mm/fault.c

+17 −10
Original line number Diff line number Diff line
@@ -54,7 +54,7 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr) 
 * 32-bit mode:

 *

 *   Sometimes AMD Athlon/Opteron CPUs report invalid exceptions on prefetch.

 *   Check that here and ignore it.

 *   Check that here and ignore it.  This is AMD erratum #91.

 *

 * 64-bit mode:

 *
@@ -83,11 +83,7 @@ check_prefetch_opcode(struct pt_regs *regs, unsigned char *instr, 
#ifdef CONFIG_X86_64

	case 0x40:

		/*

		 * In AMD64 long mode 0x40..0x4F are valid REX prefixes

		 * Need to figure out under what instruction mode the

		 * instruction was issued. Could check the LDT for lm,

		 * but for now it's good enough to assume that long

		 * mode only uses well known segments or kernel.

		 * In 64-bit mode 0x40..0x4F are valid REX prefixes

		 */

		return (!user_mode(regs) || user_64bit_mode(regs));

#endif
@@ -127,20 +123,31 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr) 
	instr = (void *)convert_ip_to_linear(current, regs);

	max_instr = instr + 15;



	if (user_mode(regs) && instr >= (unsigned char *)TASK_SIZE_MAX)

		return 0;

	/*

	 * This code has historically always bailed out if IP points to a

	 * not-present page (e.g. due to a race).  No one has ever

	 * complained about this.

	 */

	pagefault_disable();



	while (instr < max_instr) {

		unsigned char opcode;



		if (user_mode(regs)) {

			if (get_user(opcode, instr))

				break;

		} else {

			if (get_kernel_nofault(opcode, instr))

				break;

		}



		instr++;



		if (!check_prefetch_opcode(regs, instr, opcode, &prefetch))

			break;

	}



	pagefault_enable();

	return prefetch;

}