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

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Date:   Wed, 29 Aug 2018 15:55:32 +0200
From:   Jann Horn <jannh@...gle.com>
To:     bp@...e.de
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>, security@...nel.org
Subject: Re: [PATCH v2] x86/dumpstack: don't dump kernel memory based on
 usermode RIP

On Wed, Aug 29, 2018 at 9:10 AM Borislav Petkov <bp@...e.de> wrote:
>
> On Tue, Aug 28, 2018 at 05:49:01PM +0200, Jann Horn 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@...r.kernel.org
> > Fixes: 7cccf0725cf7 ("x86/dumpstack: Add a show_ip() function")
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > Signed-off-by: Jann Horn <jannh@...gle.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);
>
> Just a nitpick:
>
> <--- newline here.

The code below this point is still part of the declarations. Do you
want a newline here anyway? If you say yes, I'll adjust and resend.

> > +     /*
> > +      * 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);
>
> Other than that:
>
> Reviewed-by: Borislav Petkov <bp@...e.de>
>
> Thx.
>
> --
> Regards/Gruss,
>     Boris.
>
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
> --