Commit 42fea0a3 authored by Alexander Shishkin's avatar Alexander Shishkin Committed by Dave Hansen
Browse files

x86/traps: Communicate a LASS violation in #GP message 



A LASS violation typically results in a #GP. With LASS active, any
invalid access to user memory (including the first page frame) would be
reported as a #GP, instead of a #PF.

Unfortunately, the #GP error messages provide limited information about
the cause of the fault. This could be confusing for kernel developers
and users who are accustomed to the friendly #PF messages.

To make the transition easier, enhance the #GP Oops message to include a
hint about LASS violations. Also, add a special hint for kernel NULL
pointer dereferences to match with the existing #PF message.

Signed-off-by: default avatarAlexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarSohil Mehta <sohil.mehta@intel.com>
Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251118182911.2983253-7-sohil.mehta%40intel.com
parent 731d4375

arch/x86/kernel/traps.c

+33 −11
Original line number Diff line number Diff line
@@ -635,13 +635,23 @@ DEFINE_IDTENTRY(exc_bounds) 
enum kernel_gp_hint {

	GP_NO_HINT,

	GP_NON_CANONICAL,

	GP_CANONICAL

	GP_CANONICAL,

	GP_LASS_VIOLATION,

	GP_NULL_POINTER,

};



static const char * const kernel_gp_hint_help[] = {

	[GP_NON_CANONICAL]	= "probably for non-canonical address",

	[GP_CANONICAL]		= "maybe for address",

	[GP_LASS_VIOLATION]	= "probably LASS violation for address",

	[GP_NULL_POINTER]	= "kernel NULL pointer dereference",

};



/*

 * When an uncaught #GP occurs, try to determine the memory address accessed by

 * the instruction and return that address to the caller. Also, try to figure

 * out whether any part of the access to that address was non-canonical.

 * out whether any part of the access to that address was non-canonical or

 * across privilege levels.

 */

static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,

						 unsigned long *addr)
@@ -663,14 +673,28 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs, 
		return GP_NO_HINT;



#ifdef CONFIG_X86_64

	/* Operand is in the kernel half */

	if (*addr >= ~__VIRTUAL_MASK)

		return GP_CANONICAL;



	/* The last byte of the operand is not in the user canonical half */

	if (*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)

		return GP_NON_CANONICAL;



	/*

	 * Check that:

	 *  - the operand is not in the kernel half

	 *  - the last byte of the operand is not in the user canonical half

	 * A NULL pointer dereference usually causes a #PF. However, it

	 * can result in a #GP when LASS is active. Provide the same

	 * hint in the rare case that the condition is hit without LASS.

	 */

	if (*addr < ~__VIRTUAL_MASK &&

	    *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)

		return GP_NON_CANONICAL;

	if (*addr < PAGE_SIZE)

		return GP_NULL_POINTER;



	/*

	 * Assume that LASS caused the exception, because the address is

	 * canonical and in the user half.

	 */

	if (cpu_feature_enabled(X86_FEATURE_LASS))

		return GP_LASS_VIOLATION;

#endif



	return GP_CANONICAL;
@@ -833,9 +857,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection) 


	if (hint != GP_NO_HINT)

		snprintf(desc, sizeof(desc), GPFSTR ", %s 0x%lx",

			 (hint == GP_NON_CANONICAL) ? "probably for non-canonical address"

						    : "maybe for address",

			 gp_addr);

			 kernel_gp_hint_help[hint], gp_addr);



	/*

	 * KASAN is interested only in the non-canonical case, clear it