x86/dumpstack: Don't dump kernel memory based on usermode RIP

show_opcodes() is used both for dumping kernel instructions and for dumping
user instructions. If userspace causes #PF by jumping to a kernel address,
show_opcodes() can be reached with regs->ip controlled by the user,
pointing to kernel code. Make sure that userspace can't trick us into
dumping kernel memory into dmesg.

Fixes: 7cccf0725cf7 ("x86/dumpstack: Add a show_ip() function")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: security@kernel.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180828154901.112726-1-jannh@google.com

diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
index b6dc698..f335aad 100644 (file)
--- a/arch/x86/include/asm/stacktrace.h
+++ b/arch/x86/include/asm/stacktrace.h
@@ -111,6 +111,6 @@ static inline unsigned long caller_frame_pointer(void)
        return (unsigned long)frame;
 }
 
-void show_opcodes(u8 *rip, const char *loglvl);
+void show_opcodes(struct pt_regs *regs, const char *loglvl);
 void show_ip(struct pt_regs *regs, const char *loglvl);
 #endif /* _ASM_X86_STACKTRACE_H */

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 1596e6b..f568951 100644 (file)
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -90,14 +90,24 @@ static void printk_stack_address(unsigned long address, int reliable,
  * Thus, the 2/3rds prologue and 64 byte OPCODE_BUFSIZE is just a random
  * guesstimate in attempt to achieve all of the above.
  */
-void show_opcodes(u8 *rip, const char *loglvl)
+void show_opcodes(struct pt_regs *regs, const char *loglvl)
 {
 #define PROLOGUE_SIZE 42
 #define EPILOGUE_SIZE 21
 #define OPCODE_BUFSIZE (PROLOGUE_SIZE + 1 + EPILOGUE_SIZE)
        u8 opcodes[OPCODE_BUFSIZE];
+       unsigned long prologue = regs->ip - PROLOGUE_SIZE;
+       bool bad_ip;
 
-       if (probe_kernel_read(opcodes, rip - PROLOGUE_SIZE, OPCODE_BUFSIZE)) {
+       /*
+        * Make sure userspace isn't trying to trick us into dumping kernel
+        * memory by pointing the userspace instruction pointer at it.
+        */
+       bad_ip = user_mode(regs) &&
+               __chk_range_not_ok(prologue, OPCODE_BUFSIZE, TASK_SIZE_MAX);
+
+       if (bad_ip || probe_kernel_read(opcodes, (u8 *)prologue,
+                                       OPCODE_BUFSIZE)) {
                printk("%sCode: Bad RIP value.\n", loglvl);
        } else {
                printk("%sCode: %" __stringify(PROLOGUE_SIZE) "ph <%02x> %"
@@ -113,7 +123,7 @@ void show_ip(struct pt_regs *regs, const char *loglvl)
 #else
        printk("%sRIP: %04x:%pS\n", loglvl, (int)regs->cs, (void *)regs->ip);
 #endif
-       show_opcodes((u8 *)regs->ip, loglvl);
+       show_opcodes(regs, loglvl);
 }
 
 void show_iret_regs(struct pt_regs *regs)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index b9123c4..47bebfe 100644 (file)
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -837,7 +837,7 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 
        printk(KERN_CONT "\n");
 
-       show_opcodes((u8 *)regs->ip, loglvl);
+       show_opcodes(regs, loglvl);
 }
 
 static void