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

[Kees Cook]

On Tue, Aug 28, 2018 at 8:18 AM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> 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: stable@xxxxxxxxxxxxxxx
> Fixes: 7cccf0725cf7 ("x86/dumpstack: Add a show_ip() function")
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>

Thanks for finding this!

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  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..5d2700c98b29 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
>



-- 
Kees Cook
Pixel Security