[RFC PATCH v2 2/4] x86/fault: Attempt to fixup unhandled #PF in vDSO before signaling

[Sean Christopherson <sean.j.christopherson@intel.com>]

Call fixup_vdso_exception() in the SIGSEGV and SIGBUS paths of the
page fault handler immediately prior to signaling.  If the fault is
fixed, return cleanly and do not generate a signal.

The goal of vDSO fixup is not to fixup all faults, nor is it to avoid
all signals, but rather to report faults directly to userspace when the
fault would otherwise directly result in a signal being sent to the
process.  For example, a page fault that points to valid user memory
that happened to be swapped out should not trigger fixup, i.e. should
not be reported to userspace.

In the SIGSEGV flow, make sure to call fixup_vdso_exception() after
the error code has been sanitized.

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/mm/fault.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 2ff25ad33233..ff1cb10858d5 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -28,6 +28,7 @@
 #include <asm/mmu_context.h>		/* vma_pkey()			*/
 #include <asm/efi.h>			/* efi_recover_from_page_fault()*/
 #include <asm/desc.h>			/* store_idt(), ...		*/
+#include <asm/vdso.h>			/* fixup_vdso_exception()	*/
 
 #define CREATE_TRACE_POINTS
 #include <asm/trace/exceptions.h>
@@ -928,6 +929,9 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 		if (address >= TASK_SIZE_MAX)
 			error_code |= X86_PF_PROT;
 
+		if (fixup_vdso_exception(regs, X86_TRAP_PF, error_code, address))
+			return;
+
 		if (likely(show_unhandled_signals))
 			show_signal_msg(regs, error_code, address, tsk);
 
@@ -1045,6 +1049,9 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
 	if (is_prefetch(regs, error_code, address))
 		return;
 
+	if (fixup_vdso_exception(regs, X86_TRAP_PF, error_code, address))
+		return;
+
 	set_signal_archinfo(address, error_code);
 
 #ifdef CONFIG_MEMORY_FAILURE
-- 
2.19.2