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: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees 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

3 files changed, 27 insertions, 9 deletions

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index fedd7628f3a6..9c58ab807aeb 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -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.
diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
index b986b2ca688a..ab60a71a8dcb 100644
--- a/arch/x86/include/asm/vsyscall.h
+++ b/arch/x86/include/asm/vsyscall.h
@@ -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;
 }
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 46df4c6aae46..288a5462076f 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -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