On Fri, Jul 18, 2014 at 03:51:31PM -0700, Omar Sandoval wrote:
> Don't break into kgdb when userspace executes the kernel break instructions
> (KGDB_BREAKINST and KGDB_COMPILED_BREAK). The kernel will oops in
> kgdb_handle_exception.
>
> Signed-off-by: Omar Sandoval <osan...@osandov.com>
> ---
> The following program will immediately cause a kernel oops:
> .globl _start
> _start:
> udf #65006 @ KGDB_BREAKINST
>
> arch/arm/kernel/kgdb.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
> index 778c2f7..a74b53c 100644
> --- a/arch/arm/kernel/kgdb.c
> +++ b/arch/arm/kernel/kgdb.c
> @@ -160,12 +160,16 @@ static int kgdb_compiled_brk_fn(struct pt_regs *regs,
> unsigned int instr)
> static struct undef_hook kgdb_brkpt_hook = {
> .instr_mask = 0xffffffff,
> .instr_val = KGDB_BREAKINST,
> + .cpsr_mask = MODE_MASK,
> + .cpsr_val = SVC_MODE,
> .fn = kgdb_brk_fn
> };
>
> static struct undef_hook kgdb_compiled_brkpt_hook = {
> .instr_mask = 0xffffffff,
> .instr_val = KGDB_COMPILED_BREAK,
> + .cpsr_mask = MODE_MASK,
> + .cpsr_val = SVC_MODE,
> .fn = kgdb_compiled_brk_fn
> };
>
> --
> 2.0.1
--
Following up/clarifying this. This only happens when the kernel is compiled
with CONFIG_KGDB. When a userspace program executes KGDB_BREAKINST or
KGDB_COMPILED_BREAK, the undef_hook for kgdb catches it. The reason in kdb_stub
defaults to KDB_REASON_OOPS, so the bug manifests itself as an oops caused by
userspace (a better description for the patch would be "Don't enter KGDB when
userspace executes kgdb break instructions"). This means that a buggy/malicious
program can take down the system just by executing an instruction.
ARM64 might have the same issue, but I don't have a board to test that on.
I verified that breaking normally (e.g., with kgdbwait or through
/proc/sysrq-trigger) still works.
—
Omar
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