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SubjectRe: [PATCH] kasan: make depot_fetch_stack more robust
2016-07-01 20:38 GMT+03:00 Dmitry Vyukov <dvyukov@google.com>:
> I've hit a GPF in depot_fetch_stack when it was given
> bogus stack handle. I think it was caused by a distant
> out-of-bounds that hit a different object, as the result
> we treated uninit garbage as stack handle. Maybe there is
> something to fix in KASAN logic, but I think it makes
> sense to make depot_fetch_stack more robust as well.
>
> Verify that the provided stack handle looks correct.
>
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> ---
> For your convenience uploaded to codereview:
> https://codereview.appspot.com/295680043
>
> ---
> include/linux/stackdepot.h | 2 +-
> lib/stackdepot.c | 21 +++++++++++++++++----
> mm/kasan/report.c | 10 ++++------
> mm/page_owner.c | 12 ++++++------
> 4 files changed, 28 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
> index 7978b3e..b2dbe02 100644
> --- a/include/linux/stackdepot.h
> +++ b/include/linux/stackdepot.h
> @@ -27,6 +27,6 @@ struct stack_trace;
>
> depot_stack_handle_t depot_save_stack(struct stack_trace *trace, gfp_t flags);
>
> -void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace);
> +bool depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace);
>
> #endif
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 53ad6c0..0982331 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -181,16 +181,29 @@ static inline struct stack_record *find_stack(struct stack_record *bucket,
> return NULL;
> }
>
> -void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace)
> +bool depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace)
> {
> union handle_parts parts = { .handle = handle };
> - void *slab = stack_slabs[parts.slabindex];
> - size_t offset = parts.offset << STACK_ALLOC_ALIGN;
> - struct stack_record *stack = slab + offset;
> + void *slab;
> + struct stack_record *stack;
>
> + if (handle == 0)
> + return false;
> + if (parts.valid != 1 || parts.slabindex >= ARRAY_SIZE(stack_slabs))
> + goto bad;
> + slab = stack_slabs[parts.slabindex];
> + if (slab == NULL)
> + goto bad;
> + stack = slab + (parts.offset << STACK_ALLOC_ALIGN);
> + if (stack->handle.handle != handle)
> + goto bad;
> trace->nr_entries = trace->max_entries = stack->size;
> trace->entries = stack->entries;
> trace->skip = 0;
> + return true;
> +bad:
> + pr_err("stackdepot: fetching bogus stack %x\n", handle);
> + return false;
> }
>
> /**
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 861b977..46e4b82 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -118,15 +118,13 @@ static inline bool init_task_stack_addr(const void *addr)
>
> static void print_track(struct kasan_track *track)
> {
> - pr_err("PID = %u\n", track->pid);
> - if (track->stack) {
> - struct stack_trace trace;
> + struct stack_trace trace;
>
> - depot_fetch_stack(track->stack, &trace);
> + pr_err("PID = %u\n", track->pid);
> + if (depot_fetch_stack(track->stack, &trace))
> print_stack_trace(&trace, 0);
> - } else {
> + else
> pr_err("(stack is not available)\n");
> - }
> }
>
> static void kasan_object_err(struct kmem_cache *cache, struct page *page,
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 8fa5083..1862f05 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -252,10 +252,11 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
> if (ret >= count)
> goto err;
>
> - depot_fetch_stack(handle, &trace);
> - ret += snprint_stack_trace(kbuf + ret, count - ret, &trace, 0);
> - if (ret >= count)
> - goto err;
> + if (depot_fetch_stack(handle, &trace)) {
> + ret += snprint_stack_trace(kbuf + ret, count - ret, &trace, 0);
> + if (ret >= count)
> + goto err;
> + }
>

I don't think that adding the kernel code to work around bugs in the
kernel code makes a lot of sense.
depot_fetch_stack() fails if invalid handler is passed, and that is a
bug. You can just add WARN_ON() in
depot_fetch_stack() if you want to detect such cases..
Note that KASAN detects corruption of object's metadata, so such check
may help only in case of
corruption page owner's data.

> if (page_ext->last_migrate_reason != -1) {
> ret += snprintf(kbuf + ret, count - ret,
> @@ -307,12 +308,11 @@ void __dump_page_owner(struct page *page)
> }
>
> handle = READ_ONCE(page_ext->handle);
> - if (!handle) {
> + if (!depot_fetch_stack(handle, &trace)) {
> pr_alert("page_owner info is not active (free page?)\n");
> return;
> }
>
> - depot_fetch_stack(handle, &trace);
> pr_alert("page allocated via order %u, migratetype %s, gfp_mask %#x(%pGg)\n",
> page_ext->order, migratetype_names[mt], gfp_mask, &gfp_mask);
> print_stack_trace(&trace, 0);
> --
> 2.8.0.rc3.226.g39d4020
>

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