Commit 918ce325 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Thomas Gleixner
Browse files

x86/vsyscall: Show something useful on a read fault 



Just segfaulting the application when it tries to read the vsyscall page in
xonly mode is not helpful for those who need to debug it.

Emit a hint.

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jann Horn <jannh@google.com>
Link: https://lkml.kernel.org/r/8016afffe0eab497be32017ad7f6f7030dc3ba66.1561610354.git.luto@kernel.org
parent bd49e16e

arch/x86/entry/vsyscall/vsyscall_64.c

+18 −1
Original line number Diff line number Diff line
@@ -117,7 +117,8 @@ static bool write_ok_or_segv(unsigned long ptr, size_t size) 
	}

}



bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)

bool emulate_vsyscall(unsigned long error_code,

		      struct pt_regs *regs, unsigned long address)

{

	struct task_struct *tsk;

	unsigned long caller;
@@ -126,6 +127,22 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address) 
	long ret;

	unsigned long orig_dx;



	/* Write faults or kernel-privilege faults never get fixed up. */

	if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)

		return false;



	if (!(error_code & X86_PF_INSTR)) {

		/* Failed vsyscall read */

		if (vsyscall_mode == EMULATE)

			return false;



		/*

		 * User code tried and failed to read the vsyscall page.

		 */

		warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");

		return false;

	}



	/*

	 * No point in checking CS -- the only way to get here is a user mode

	 * trap to a high address, which means that we're in 64-bit user code.

arch/x86/include/asm/vsyscall.h

+4 −2
Original line number Diff line number Diff line
@@ -13,10 +13,12 @@ extern void set_vsyscall_pgtable_user_bits(pgd_t *root); 
 * Called on instruction fetch fault in vsyscall page.

 * Returns true if handled.

 */

extern bool emulate_vsyscall(struct pt_regs *regs, unsigned long address);

extern bool emulate_vsyscall(unsigned long error_code,

			     struct pt_regs *regs, unsigned long address);

#else

static inline void map_vsyscall(void) {}

static inline bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)

static inline bool emulate_vsyscall(unsigned long error_code,

				    struct pt_regs *regs, unsigned long address)

{

	return false;

}

arch/x86/mm/fault.c

+5 −6
Original line number Diff line number Diff line
@@ -1369,16 +1369,15 @@ void do_user_addr_fault(struct pt_regs *regs, 


#ifdef CONFIG_X86_64

	/*

	 * Instruction fetch faults in the vsyscall page might need

	 * emulation.  The vsyscall page is at a high address

	 * (>PAGE_OFFSET), but is considered to be part of the user

	 * address space.

	 * Faults in the vsyscall page might need emulation.  The

	 * vsyscall page is at a high address (>PAGE_OFFSET), but is

	 * considered to be part of the user address space.

	 *

	 * The vsyscall page does not have a "real" VMA, so do this

	 * emulation before we go searching for VMAs.

	 */

	if ((hw_error_code & X86_PF_INSTR) && is_vsyscall_vaddr(address)) {

		if (emulate_vsyscall(regs, address))

	if (is_vsyscall_vaddr(address)) {

		if (emulate_vsyscall(hw_error_code, regs, address))

			return;

	}

#endif

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