[PATCH v2] x86/dumpstack: don't dump kernel memory based on usermode RIP

[Jann Horn]

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.

Cc: sta...@vger.kernel.org
Fixes: 7cccf0725cf7 ("x86/dumpstack: Add a show_ip() function")
Reviewed-by: Kees Cook <keesc...@chromium.org>
Signed-off-by: Jann Horn <ja...@google.com>
---
v2: Andy pointed out that I probably shouldn't be doing wrapping
arithmetic on pointers.

 arch/x86/include/asm/stacktrace.h |  2 +-
 arch/x86/kernel/dumpstack.c       | 13 ++++++++++---
 arch/x86/mm/fault.c               |  2 +-
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/stacktrace.h 
b/arch/x86/include/asm/stacktrace.h
index b6dc698f992a..f335aad404a4 100644
--- 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 9c8652974f8e..14b337582b6f 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -89,14 +89,21 @@ 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];
+       u8 *prologue = (u8 *)(regs->ip - PROLOGUE_SIZE);
+       /*
+        * Make sure userspace isn't trying to trick us into dumping kernel
+        * memory by pointing the userspace instruction pointer at it.
+        */
+       bool bad_ip = user_mode(regs) &&
+                     __range_not_ok(prologue, OPCODE_BUFSIZE, TASK_SIZE_MAX);
 
-       if (probe_kernel_read(opcodes, rip - PROLOGUE_SIZE, OPCODE_BUFSIZE)) {
+       if (bad_ip || probe_kernel_read(opcodes, prologue, OPCODE_BUFSIZE)) {
                printk("%sCode: Bad RIP value.\n", loglvl);
        } else {
                printk("%sCode: %" __stringify(PROLOGUE_SIZE) "ph <%02x> %"
@@ -112,7 +119,7 @@ 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 b9123c497e0a..47bebfe6efa7 100644
--- 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
-- 
2.19.0.rc0.228.g281dcd1b4d0-goog