[PATCH 4.18 122/123] x86/dumpstack: Dont dump kernel memory based on usermode RIP

[Greg Kroah-Hartman]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@xxxxxxxxxx>

commit 342db04ae71273322f0011384a9ed414df8bdae4 upstream.

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@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: security@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20180828154901.112726-1-jannh@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>


---
 arch/x86/include/asm/stacktrace.h |    2 +-
 arch/x86/kernel/dumpstack.c       |   21 +++++++++++++++------
 arch/x86/mm/fault.c               |    2 +-
 3 files changed, 17 insertions(+), 8 deletions(-)

--- a/arch/x86/include/asm/stacktrace.h
+++ b/arch/x86/include/asm/stacktrace.h
@@ -111,6 +111,6 @@ static inline unsigned long caller_frame
 	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 */
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -92,23 +92,32 @@ static void printk_stack_address(unsigne
  * 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)
 {
 	unsigned int code_prologue = OPCODE_BUFSIZE * 2 / 3;
 	u8 opcodes[OPCODE_BUFSIZE];
-	u8 *ip;
+	unsigned long ip;
 	int i;
+	bool bad_ip;
 
 	printk("%sCode: ", loglvl);
 
-	ip = (u8 *)rip - code_prologue;
-	if (probe_kernel_read(opcodes, ip, OPCODE_BUFSIZE)) {
+	ip = regs->ip - code_prologue;
+
+	/*
+	 * 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(ip, OPCODE_BUFSIZE, TASK_SIZE_MAX);
+
+	if (bad_ip || probe_kernel_read(opcodes, (u8 *)ip, OPCODE_BUFSIZE)) {
 		pr_cont("Bad RIP value.\n");
 		return;
 	}
 
 	for (i = 0; i < OPCODE_BUFSIZE; i++, ip++) {
-		if (ip == rip)
+		if (ip == regs->ip)
 			pr_cont("<%02x> ", opcodes[i]);
 		else
 			pr_cont("%02x ", opcodes[i]);
@@ -123,7 +132,7 @@ void show_ip(struct pt_regs *regs, const
 #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)
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -838,7 +838,7 @@ show_signal_msg(struct pt_regs *regs, un
 
 	printk(KERN_CONT "\n");
 
-	show_opcodes((u8 *)regs->ip, loglvl);
+	show_opcodes(regs, loglvl);
 }
 
 static void