linux/arch/x86/kernel/unwind_frame.c
Josh Poimboeuf 84936118bd x86/unwind: Disable KASAN checks for non-current tasks
There are a handful of callers to save_stack_trace_tsk() and
show_stack() which try to unwind the stack of a task other than current.
In such cases, it's remotely possible that the task is running on one
CPU while the unwinder is reading its stack from another CPU, causing
the unwinder to see stack corruption.

These cases seem to be mostly harmless.  The unwinder has checks which
prevent it from following bad pointers beyond the bounds of the stack.
So it's not really a bug as long as the caller understands that
unwinding another task will not always succeed.

In such cases, it's possible that the unwinder may read a KASAN-poisoned
region of the stack.  Account for that by using READ_ONCE_NOCHECK() when
reading the stack of another task.

Use READ_ONCE() when reading the stack of the current task, since KASAN
warnings can still be useful for finding bugs in that case.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/4c575eb288ba9f73d498dfe0acde2f58674598f1.1483978430.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-01-12 09:28:27 +01:00

299 lines
7.9 KiB
C

#include <linux/sched.h>
#include <asm/ptrace.h>
#include <asm/bitops.h>
#include <asm/stacktrace.h>
#include <asm/unwind.h>
#define FRAME_HEADER_SIZE (sizeof(long) * 2)
/*
* This disables KASAN checking when reading a value from another task's stack,
* since the other task could be running on another CPU and could have poisoned
* the stack in the meantime.
*/
#define READ_ONCE_TASK_STACK(task, x) \
({ \
unsigned long val; \
if (task == current) \
val = READ_ONCE(x); \
else \
val = READ_ONCE_NOCHECK(x); \
val; \
})
static void unwind_dump(struct unwind_state *state, unsigned long *sp)
{
static bool dumped_before = false;
bool prev_zero, zero = false;
unsigned long word;
if (dumped_before)
return;
dumped_before = true;
printk_deferred("unwind stack type:%d next_sp:%p mask:%lx graph_idx:%d\n",
state->stack_info.type, state->stack_info.next_sp,
state->stack_mask, state->graph_idx);
for (sp = state->orig_sp; sp < state->stack_info.end; sp++) {
word = READ_ONCE_NOCHECK(*sp);
prev_zero = zero;
zero = word == 0;
if (zero) {
if (!prev_zero)
printk_deferred("%p: %016x ...\n", sp, 0);
continue;
}
printk_deferred("%p: %016lx (%pB)\n", sp, word, (void *)word);
}
}
unsigned long unwind_get_return_address(struct unwind_state *state)
{
unsigned long addr;
unsigned long *addr_p = unwind_get_return_address_ptr(state);
if (unwind_done(state))
return 0;
if (state->regs && user_mode(state->regs))
return 0;
addr = READ_ONCE_TASK_STACK(state->task, *addr_p);
addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, addr,
addr_p);
return __kernel_text_address(addr) ? addr : 0;
}
EXPORT_SYMBOL_GPL(unwind_get_return_address);
static size_t regs_size(struct pt_regs *regs)
{
/* x86_32 regs from kernel mode are two words shorter: */
if (IS_ENABLED(CONFIG_X86_32) && !user_mode(regs))
return sizeof(*regs) - 2*sizeof(long);
return sizeof(*regs);
}
static bool is_last_task_frame(struct unwind_state *state)
{
unsigned long bp = (unsigned long)state->bp;
unsigned long regs = (unsigned long)task_pt_regs(state->task);
/*
* We have to check for the last task frame at two different locations
* because gcc can occasionally decide to realign the stack pointer and
* change the offset of the stack frame by a word in the prologue of a
* function called by head/entry code.
*/
return bp == regs - FRAME_HEADER_SIZE ||
bp == regs - FRAME_HEADER_SIZE - sizeof(long);
}
/*
* This determines if the frame pointer actually contains an encoded pointer to
* pt_regs on the stack. See ENCODE_FRAME_POINTER.
*/
static struct pt_regs *decode_frame_pointer(unsigned long *bp)
{
unsigned long regs = (unsigned long)bp;
if (!(regs & 0x1))
return NULL;
return (struct pt_regs *)(regs & ~0x1);
}
static bool update_stack_state(struct unwind_state *state, void *addr,
size_t len)
{
struct stack_info *info = &state->stack_info;
enum stack_type orig_type = info->type;
/*
* If addr isn't on the current stack, switch to the next one.
*
* We may have to traverse multiple stacks to deal with the possibility
* that 'info->next_sp' could point to an empty stack and 'addr' could
* be on a subsequent stack.
*/
while (!on_stack(info, addr, len))
if (get_stack_info(info->next_sp, state->task, info,
&state->stack_mask))
return false;
if (!state->orig_sp || info->type != orig_type)
state->orig_sp = addr;
return true;
}
bool unwind_next_frame(struct unwind_state *state)
{
struct pt_regs *regs;
unsigned long *next_bp, *next_frame;
size_t next_len;
enum stack_type prev_type = state->stack_info.type;
if (unwind_done(state))
return false;
/* have we reached the end? */
if (state->regs && user_mode(state->regs))
goto the_end;
if (is_last_task_frame(state)) {
regs = task_pt_regs(state->task);
/*
* kthreads (other than the boot CPU's idle thread) have some
* partial regs at the end of their stack which were placed
* there by copy_thread_tls(). But the regs don't have any
* useful information, so we can skip them.
*
* This user_mode() check is slightly broader than a PF_KTHREAD
* check because it also catches the awkward situation where a
* newly forked kthread transitions into a user task by calling
* do_execve(), which eventually clears PF_KTHREAD.
*/
if (!user_mode(regs))
goto the_end;
/*
* We're almost at the end, but not quite: there's still the
* syscall regs frame. Entry code doesn't encode the regs
* pointer for syscalls, so we have to set it manually.
*/
state->regs = regs;
state->bp = NULL;
return true;
}
/* get the next frame pointer */
if (state->regs)
next_bp = (unsigned long *)state->regs->bp;
else
next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task,*state->bp);
/* is the next frame pointer an encoded pointer to pt_regs? */
regs = decode_frame_pointer(next_bp);
if (regs) {
next_frame = (unsigned long *)regs;
next_len = sizeof(*regs);
} else {
next_frame = next_bp;
next_len = FRAME_HEADER_SIZE;
}
/* make sure the next frame's data is accessible */
if (!update_stack_state(state, next_frame, next_len)) {
/*
* Don't warn on bad regs->bp. An interrupt in entry code
* might cause a false positive warning.
*/
if (state->regs)
goto the_end;
goto bad_address;
}
/* Make sure it only unwinds up and doesn't overlap the last frame: */
if (state->stack_info.type == prev_type) {
if (state->regs && (void *)next_frame < (void *)state->regs + regs_size(state->regs))
goto bad_address;
if (state->bp && (void *)next_frame < (void *)state->bp + FRAME_HEADER_SIZE)
goto bad_address;
}
/* move to the next frame */
if (regs) {
state->regs = regs;
state->bp = NULL;
} else {
state->bp = next_bp;
state->regs = NULL;
}
return true;
bad_address:
/*
* When unwinding a non-current task, the task might actually be
* running on another CPU, in which case it could be modifying its
* stack while we're reading it. This is generally not a problem and
* can be ignored as long as the caller understands that unwinding
* another task will not always succeed.
*/
if (state->task != current)
goto the_end;
if (state->regs) {
printk_deferred_once(KERN_WARNING
"WARNING: kernel stack regs at %p in %s:%d has bad 'bp' value %p\n",
state->regs, state->task->comm,
state->task->pid, next_frame);
unwind_dump(state, (unsigned long *)state->regs);
} else {
printk_deferred_once(KERN_WARNING
"WARNING: kernel stack frame pointer at %p in %s:%d has bad value %p\n",
state->bp, state->task->comm,
state->task->pid, next_frame);
unwind_dump(state, state->bp);
}
the_end:
state->stack_info.type = STACK_TYPE_UNKNOWN;
return false;
}
EXPORT_SYMBOL_GPL(unwind_next_frame);
void __unwind_start(struct unwind_state *state, struct task_struct *task,
struct pt_regs *regs, unsigned long *first_frame)
{
unsigned long *bp, *frame;
size_t len;
memset(state, 0, sizeof(*state));
state->task = task;
/* don't even attempt to start from user mode regs */
if (regs && user_mode(regs)) {
state->stack_info.type = STACK_TYPE_UNKNOWN;
return;
}
/* set up the starting stack frame */
bp = get_frame_pointer(task, regs);
regs = decode_frame_pointer(bp);
if (regs) {
state->regs = regs;
frame = (unsigned long *)regs;
len = sizeof(*regs);
} else {
state->bp = bp;
frame = bp;
len = FRAME_HEADER_SIZE;
}
/* initialize stack info and make sure the frame data is accessible */
get_stack_info(frame, state->task, &state->stack_info,
&state->stack_mask);
update_stack_state(state, frame, len);
/*
* The caller can provide the address of the first frame directly
* (first_frame) or indirectly (regs->sp) to indicate which stack frame
* to start unwinding at. Skip ahead until we reach it.
*/
while (!unwind_done(state) &&
(!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
state->bp < first_frame))
unwind_next_frame(state);
}
EXPORT_SYMBOL_GPL(__unwind_start);