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Subject[PATCH 4.18 122/123] x86/dumpstack: Dont dump kernel memory based on usermode RIP
Date
4.18-stable review patch.  If anyone has any objections, please let me know.

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

From: Jann Horn <jannh@google.com>

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@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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


---
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

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