+cc x86 folks

On Thu, Oct 18, 2018 at 1:18 PM Christophe LEROY
[off-list ref] wrote:

Le 18/10/2018 à 13:12, Jann Horn a écrit :

quoted

On Thu, Oct 18, 2018 at 11:28 AM Christophe LEROY
[off-list ref] wrote:

quoted

Le 05/10/2018 à 15:21, Michael Ellerman a écrit :

quoted

Recently we implemented show_user_instructions() which dumps the code
around the NIP when a user space process dies with an unhandled
signal. This was modelled on the x86 code, and we even went so far as
to implement the exact same bug, namely that if the user process
crashed with its NIP pointing into the kernel we will dump kernel text
to dmesg. eg:

    bad-bctr[2996]: segfault (11) at c000000000010000 nip c000000000010000 lr 12d0b0894 code 1
    bad-bctr[2996]: code: fbe10068 7cbe2b78 7c7f1b78 fb610048 38a10028 38810020 fb810050 7f8802a6
    bad-bctr[2996]: code: 3860001c f8010080 48242371 60000000 <7c7b1b79> 4082002c e8010080 eb610048

This was discovered on x86 by Jann Horn and fixed in commit
342db04ae712 ("x86/dumpstack: Don't dump kernel memory based on usermode RIP").

Fix it by checking the adjusted NIP value (pc) and number of
instructions against USER_DS, and bail if we fail the check, eg:

    bad-bctr[2969]: segfault (11) at c000000000010000 nip c000000000010000 lr 107930894 code 1
    bad-bctr[2969]: Bad NIP, not dumping instructions.

Fixes: 88b0fe175735 ("powerpc: Add show_user_instructions()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
   arch/powerpc/kernel/process.c | 10 ++++++++++
   1 file changed, 10 insertions(+)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 913c5725cdb2..bb6ac471a784 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c

@@ -1306,6 +1306,16 @@ void show_user_instructions(struct pt_regs *regs)

       pc = regs->nip - (instructions_to_print * 3 / 4 * sizeof(int));

+     /*
+      * Make sure the NIP points at userspace, not kernel text/data or
+      * elsewhere.
+      */
+     if (!__access_ok(pc, instructions_to_print * sizeof(int), USER_DS)) {
+             pr_info("%s[%d]: Bad NIP, not dumping instructions.\n",
+                     current->comm, current->pid);
+             return;
+     }
+

Is there any reason for not using access_ok() here ?

It's probably more robust this way, in case someone decides to call
into this from kernel exception context at some point, or something
like that?

But access_ok() uses current->thread.addr_limit, while USER_DS may
provide a larger segment:

#ifdef __powerpc64__
/* We use TASK_SIZE_USER64 as TASK_SIZE is not constant */
#define USER_DS         MAKE_MM_SEG(TASK_SIZE_USER64 - 1)
#else
#define USER_DS         MAKE_MM_SEG(TASK_SIZE - 1)
#endif

Where do you write a smaller value than USER_DS into the addr_limit?

The kernel is full of places that assume that any access up to USER_DS
is safe; for example, perf_output_sample_ustack(),
get_perf_callchain(), do_exit(), flush_old_exec(), vma_dump_size(),
... - and I also don't see anything in the powerpc code that would
ever write a smaller value into the addr_limit.

I don't know powerpc well, but AFAIK the rule on X86 is basically that
even for compat tasks, attempting to access anything up to USER_DS is
safe because the kernel doesn't put any kernel mappings there. Is that
different on powerpc?