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

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index fedd762..9c58ab8 100644 (file)
--- 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 b986b2c..ab60a71 100644 (file)
--- 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 46df4c6..288a546 100644 (file)
--- 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