Commit 9f5cb6b3 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Ingo Molnar
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x86/ldt: Make the LDT mapping RO 



Now that the LDT mapping is in a known area when PAGE_TABLE_ISOLATION is
enabled its a primary target for attacks, if a user space interface fails
to validate a write address correctly. That can never happen, right?

The SDM states:

    If the segment descriptors in the GDT or an LDT are placed in ROM, the
    processor can enter an indefinite loop if software or the processor
    attempts to update (write to) the ROM-based segment descriptors. To
    prevent this problem, set the accessed bits for all segment descriptors
    placed in a ROM. Also, remove operating-system or executive code that
    attempts to modify segment descriptors located in ROM.

So its a valid approach to set the ACCESS bit when setting up the LDT entry
and to map the table RO. Fixup the selftest so it can handle that new mode.

Remove the manual ACCESS bit setter in set_tls_desc() as this is now
pointless. Folded the patch from Peter Ziljstra.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent a4b51ef6

arch/x86/include/asm/desc.h

+2 −0
Original line number Diff line number Diff line
@@ -21,6 +21,8 @@ static inline void fill_ldt(struct desc_struct *desc, const struct user_desc *in 


	desc->type		= (info->read_exec_only ^ 1) << 1;

	desc->type	       |= info->contents << 2;

	/* Set the ACCESS bit so it can be mapped RO */

	desc->type	       |= 1;



	desc->s			= 1;

	desc->dpl		= 0x3;

arch/x86/kernel/ldt.c

+6 −1
Original line number Diff line number Diff line
@@ -158,7 +158,12 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot) 
		ptep = get_locked_pte(mm, va, &ptl);

		if (!ptep)

			return -ENOMEM;

		pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL & ~_PAGE_GLOBAL));

		/*

		 * Map it RO so the easy to find address is not a primary

		 * target via some kernel interface which misses a

		 * permission check.

		 */

		pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL));

		set_pte_at(mm, va, ptep, pte);

		pte_unmap_unlock(ptep, ptl);

	}

arch/x86/kernel/tls.c

+2 −9
Original line number Diff line number Diff line
@@ -93,17 +93,10 @@ static void set_tls_desc(struct task_struct *p, int idx, 
	cpu = get_cpu();



	while (n-- > 0) {

		if (LDT_empty(info) || LDT_zero(info)) {

		if (LDT_empty(info) || LDT_zero(info))

			memset(desc, 0, sizeof(*desc));

		} else {

		else

			fill_ldt(desc, info);



			/*

			 * Always set the accessed bit so that the CPU

			 * doesn't try to write to the (read-only) GDT.

			 */

			desc->type |= 1;

		}

		++info;

		++desc;

	}

tools/testing/selftests/x86/ldt_gdt.c

+1 −2
Original line number Diff line number Diff line
@@ -122,8 +122,7 @@ static void check_valid_segment(uint16_t index, int ldt, 
	 * NB: Different Linux versions do different things with the

	 * accessed bit in set_thread_area().

	 */

	if (ar != expected_ar &&

	    (ldt || ar != (expected_ar | AR_ACCESSED))) {

	if (ar != expected_ar && ar != (expected_ar | AR_ACCESSED)) {

		printf("[FAIL]\t%s entry %hu has AR 0x%08X but expected 0x%08X\n",

		       (ldt ? "LDT" : "GDT"), index, ar, expected_ar);

		nerrs++;

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