Re: [PATCH] kmemleak: fix kmemleak false positive report with HW tag-based kasan enable

[Kuan-Ying Lee]

+Cc kasan group

On Thu, 2021-11-18 at 13:44 +0800, Kuan-Ying Lee wrote:
> With HW tag-based kasan enable, We will get the warning
> when we free object whose address starts with 0xFF.
>
> It is because kmemleak rbtree stores tagged object and
> this freeing object's tag does not match with rbtree object.
>
> In the example below, kmemleak rbtree stores the tagged object in
> the kmalloc(), and kfree() gets the pointer with 0xFF tag.
>
> Call sequence:
> ptr = kmalloc(size, GFP_KERNEL);
> page = virt_to_page(ptr);
> kfree(page_address(page));
> ptr = kmalloc(size, GFP_KERNEL);
>
> Call sequence like that may cause the warning as following:
> 1) Freeing unknown object:
> In kfree(), we will get free unknown object warning in
> kmemleak_free().
> Because object(0xFx) in kmemleak rbtree and pointer(0xFF) in kfree()
> have
> different tag.
>
> 2) Overlap existing:
> When we allocate that object with the same hw-tag again, we will
> find the overlap in the kmemleak rbtree and kmemleak thread will
> be killed.
>
> [ 116.685312] kmemleak: Freeing unknown object at 0xffff000003f88000
> [ 116.686422] CPU: 5 PID: 177 Comm: cat Not tainted 5.16.0-rc1-dirty
> #21
> [ 116.687067] Hardware name: linux,dummy-virt (DT)
> [ 116.687496] Call trace:
> [ 116.687792] dump_backtrace+0x0/0x1ac
> [ 116.688255] show_stack+0x1c/0x30
> [ 116.688663] dump_stack_lvl+0x68/0x84
> [ 116.689096] dump_stack+0x1c/0x38
> [ 116.689499] kmemleak_free+0x6c/0x70
> [ 116.689919] slab_free_freelist_hook+0x104/0x200
> [ 116.690420] kmem_cache_free+0xa8/0x3d4
> [ 116.690845] test_version_show+0x270/0x3a0
> [ 116.691344] module_attr_show+0x28/0x40
> [ 116.691789] sysfs_kf_seq_show+0xb0/0x130
> [ 116.692245] kernfs_seq_show+0x30/0x40
> [ 116.692678] seq_read_iter+0x1bc/0x4b0
> [ 116.692678] seq_read_iter+0x1bc/0x4b0
> [ 116.693114] kernfs_fop_read_iter+0x144/0x1c0
> [ 116.693586] generic_file_splice_read+0xd0/0x184
> [ 116.694078] do_splice_to+0x90/0xe0
> [ 116.694498] splice_direct_to_actor+0xb8/0x250
> [ 116.694975] do_splice_direct+0x88/0xd4
> [ 116.695409] do_sendfile+0x2b0/0x344
> [ 116.695829] __arm64_sys_sendfile64+0x164/0x16c
> [ 116.696306] invoke_syscall+0x48/0x114
> [ 116.696735] el0_svc_common.constprop.0+0x44/0xec
> [ 116.697263] do_el0_svc+0x74/0x90
> [ 116.697665] el0_svc+0x20/0x80
> [ 116.698261] el0t_64_sync_handler+0x1a8/0x1b0
> [ 116.698695] el0t_64_sync+0x1ac/0x1b0
> ...
> [ 117.520301] kmemleak: Cannot insert 0xf2ff000003f88000 into the
> object search tree (overlaps existing)
> [ 117.521118] CPU: 5 PID: 178 Comm: cat Not tainted 5.16.0-rc1-dirty
> #21
> [ 117.521827] Hardware name: linux,dummy-virt (DT)
> [ 117.522287] Call trace:
> [ 117.522586] dump_backtrace+0x0/0x1ac
> [ 117.523053] show_stack+0x1c/0x30
> [ 117.523578] dump_stack_lvl+0x68/0x84
> [ 117.524039] dump_stack+0x1c/0x38
> [ 117.524472] create_object.isra.0+0x2d8/0x2fc
> [ 117.524975] kmemleak_alloc+0x34/0x40
> [ 117.525416] kmem_cache_alloc+0x23c/0x2f0
> [ 117.525914] test_version_show+0x1fc/0x3a0
> [ 117.526379] module_attr_show+0x28/0x40
> [ 117.526827] sysfs_kf_seq_show+0xb0/0x130
> [ 117.527363] kernfs_seq_show+0x30/0x40
> [ 117.527848] seq_read_iter+0x1bc/0x4b0
> [ 117.528320] kernfs_fop_read_iter+0x144/0x1c0
> [ 117.528809] generic_file_splice_read+0xd0/0x184
> [ 117.529316] do_splice_to+0x90/0xe0
> [ 117.529734] splice_direct_to_actor+0xb8/0x250
> [ 117.530227] do_splice_direct+0x88/0xd4
> [ 117.530686] do_sendfile+0x2b0/0x344
> [ 117.531154] __arm64_sys_sendfile64+0x164/0x16c
> [ 117.531673] invoke_syscall+0x48/0x114
> [ 117.532111] el0_svc_common.constprop.0+0x44/0xec
> [ 117.532621] do_el0_svc+0x74/0x90
> [ 117.533048] el0_svc+0x20/0x80
> [ 117.533461] el0t_64_sync_handler+0x1a8/0x1b0
> [ 117.533950] el0t_64_sync+0x1ac/0x1b0
> [ 117.534625] kmemleak: Kernel memory leak detector disabled
> [ 117.535201] kmemleak: Object 0xf2ff000003f88000 (size 128):
> [ 117.535761] kmemleak: comm "cat", pid 177, jiffies 4294921177
> [ 117.536339] kmemleak: min_count = 1
> [ 117.536718] kmemleak: count = 0
> [ 117.537068] kmemleak: flags = 0x1
> [ 117.537429] kmemleak: checksum = 0
> [ 117.537806] kmemleak: backtrace:
> [ 117.538211] kmem_cache_alloc+0x23c/0x2f0
> [ 117.538924] test_version_show+0x1fc/0x3a0
> [ 117.539393] module_attr_show+0x28/0x40
> [ 117.539844] sysfs_kf_seq_show+0xb0/0x130
> [ 117.540304] kernfs_seq_show+0x30/0x40
> [ 117.540750] seq_read_iter+0x1bc/0x4b0
> [ 117.541206] kernfs_fop_read_iter+0x144/0x1c0
> [ 117.541687] generic_file_splice_read+0xd0/0x184
> [ 117.542182] do_splice_to+0x90/0xe0
> [ 117.542611] splice_direct_to_actor+0xb8/0x250
> [ 117.543097] do_splice_direct+0x88/0xd4
> [ 117.543544] do_sendfile+0x2b0/0x344
> [ 117.543983] __arm64_sys_sendfile64+0x164/0x16c
> [ 117.544471] invoke_syscall+0x48/0x114
> [ 117.544917] el0_svc_common.constprop.0+0x44/0xec
> [ 117.545416] do_el0_svc+0x74/0x90
> [ 117.554100] kmemleak: Automatic memory scanning thread ended
>
> Signed-off-by: Kuan-Ying Lee <Kuan-Y...@mediatek.com>
> ---
> mm/kmemleak.c | 17 ++++++++++++-----
> 1 file changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index b57383c17cf6..fa12e2e08cdc 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -381,15 +381,20 @@ static void dump_object_info(struct
> kmemleak_object *object)
> static struct kmemleak_object *lookup_object(unsigned long ptr, int
> alias)
> {
> struct rb_node *rb = object_tree_root.rb_node;
> + unsigned long untagged_ptr = (unsigned
> long)kasan_reset_tag((void *)ptr);
>
> while (rb) {
> struct kmemleak_object *object =
> rb_entry(rb, struct kmemleak_object, rb_node);
> - if (ptr < object->pointer)
> + unsigned long untagged_objp;
> +
> + untagged_objp = (unsigned long)kasan_reset_tag((void
> *)object->pointer);
> +
> + if (untagged_ptr < untagged_objp)
> rb = object->rb_node.rb_left;
> - else if (object->pointer + object->size <= ptr)
> + else if (untagged_objp + object->size <= untagged_ptr)
> rb = object->rb_node.rb_right;
> - else if (object->pointer == ptr || alias)
> + else if (untagged_objp == untagged_ptr || alias)
> return object;
> else {
> kmemleak_warn("Found object by alias at
> 0x%08lx\n",
> @@ -576,6 +581,7 @@ static struct kmemleak_object
> *create_object(unsigned long ptr, size_t size,
> struct kmemleak_object *object, *parent;
> struct rb_node **link, *rb_parent;
> unsigned long untagged_ptr;
> + unsigned long untagged_objp;
>
> object = mem_pool_alloc(gfp);
> if (!object) {
> @@ -629,9 +635,10 @@ static struct kmemleak_object
> *create_object(unsigned long ptr, size_t size,
> while (*link) {
> rb_parent = *link;
> parent = rb_entry(rb_parent, struct kmemleak_object,
> rb_node);
> - if (ptr + size <= parent->pointer)
> + untagged_objp = (unsigned long)kasan_reset_tag((void
> *)parent->pointer);
> + if (untagged_ptr + size <= untagged_objp)
> link = &parent->rb_node.rb_left;
> - else if (parent->pointer + parent->size <= ptr)
> + else if (untagged_objp + parent->size <= untagged_ptr)
> link = &parent->rb_node.rb_right;
> else {
> kmemleak_stop("Cannot insert 0x%lx into the
> object search tree (overlaps existing)\n",
> --
> 2.18.0
>

[Andrey Konovalov]

On Thu, Nov 18, 2021 at 10:20 AM Kuan-Ying Lee
<Kuan-Y...@mediatek.com> wrote:
>
> +Cc kasan group
>
> On Thu, 2021-11-18 at 13:44 +0800, Kuan-Ying Lee wrote:
> > With HW tag-based kasan enable, We will get the warning
> > when we free object whose address starts with 0xFF.
> >
> > It is because kmemleak rbtree stores tagged object and
> > this freeing object's tag does not match with rbtree object.
> >
> > In the example below, kmemleak rbtree stores the tagged object in
> > the kmalloc(), and kfree() gets the pointer with 0xFF tag.
> >
> > Call sequence:
> > ptr = kmalloc(size, GFP_KERNEL);
> > page = virt_to_page(ptr);
> > kfree(page_address(page));
> > ptr = kmalloc(size, GFP_KERNEL);

How is this call sequence valid? page_address returns the address of
the start of the page, while kmalloced object could have been located
in the middle of it.

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[Kuan-Ying Lee]

On Fri, 2021-11-19 at 22:15 +0800, Andrey Konovalov wrote:
> On Thu, Nov 18, 2021 at 10:20 AM Kuan-Ying Lee
> <Kuan-Y...@mediatek.com> wrote:
> >
> > +Cc kasan group
> >
> > On Thu, 2021-11-18 at 13:44 +0800, Kuan-Ying Lee wrote:
> > > With HW tag-based kasan enable, We will get the warning
> > > when we free object whose address starts with 0xFF.
> > >
> > > It is because kmemleak rbtree stores tagged object and
> > > this freeing object's tag does not match with rbtree object.
> > >
> > > In the example below, kmemleak rbtree stores the tagged object in
> > > the kmalloc(), and kfree() gets the pointer with 0xFF tag.
> > >
> > > Call sequence:
> > > ptr = kmalloc(size, GFP_KERNEL);
> > > page = virt_to_page(ptr);
> > > kfree(page_address(page));
> > > ptr = kmalloc(size, GFP_KERNEL);
>
> How is this call sequence valid? page_address returns the address of
> the start of the page, while kmalloced object could have been located
> in the middle of it.

Thanks for pointing out. I miss the offset.

It should be listed as below.

ptr = kmalloc(size, GFP_KERNEL);
page = virt_to_page(ptr);

offset = offset_in_page(ptr);
kfree(page_address(page) + offset);
ptr = kmalloc(size, GFP_KERNEL);

> > https://urldefense.com/v3/__https://groups.google.com/d/msgid/kasan-dev/754511d9a0368065768cc3ad8037184d62c3fbd1.camel*40mediatek.com__;JQ!!CTRNKA9wMg0ARbw!y7gGU0PsiMId4XiGTZzBUUL_WtWQ24nnTQtGbFrZ46wqfwk8ZeYCVP2pYmNFHrAUGOKr1g$
> > .

[Andrew Morton]

On Fri, 19 Nov 2021 23:12:55 +0800 Kuan-Ying Lee <Kuan-Y...@mediatek.com> wrote:

> > > > Call sequence:
> > > > ptr = kmalloc(size, GFP_KERNEL);
> > > > page = virt_to_page(ptr);
> > > > kfree(page_address(page));
> > > > ptr = kmalloc(size, GFP_KERNEL);
> >
> > How is this call sequence valid? page_address returns the address of
> > the start of the page, while kmalloced object could have been located
> > in the middle of it.
>
> Thanks for pointing out. I miss the offset.
>
> It should be listed as below.
>
> ptr = kmalloc(size, GFP_KERNEL);
> page = virt_to_page(ptr);
> offset = offset_in_page(ptr);
> kfree(page_address(page) + offset);
> ptr = kmalloc(size, GFP_KERNEL);

I updated the changelog to reflect this.

[Kuan-Ying Lee]

Thanks for updating changelog. :)

[Andrey Konovalov]

On Thu, Nov 18, 2021 at 10:20 AM Kuan-Ying Lee
<Kuan-Y...@mediatek.com> wrote:
>

The two lines above can be squashed together.

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[Andrew Morton]

On Thu, 25 Nov 2021 17:13:36 +0100 Andrey Konovalov <andre...@gmail.com> wrote:

> > > kmemleak_object *object)
> > > static struct kmemleak_object *lookup_object(unsigned long ptr, int
> > > alias)
> > > {
> > > struct rb_node *rb = object_tree_root.rb_node;
> > > + unsigned long untagged_ptr = (unsigned
> > > long)kasan_reset_tag((void *)ptr);
> > >
> > > while (rb) {
> > > struct kmemleak_object *object =
> > > rb_entry(rb, struct kmemleak_object, rb_node);
> > > - if (ptr < object->pointer)
> > > + unsigned long untagged_objp;
> > > +
> > > + untagged_objp = (unsigned long)kasan_reset_tag((void
> > > *)object->pointer);
>
> The two lines above can be squashed together.

That would make a too-long line even longer. In fact I think it's
better to go the other way:

--- a/mm/kmemleak.c~kmemleak-fix-kmemleak-false-positive-report-with-hw-tag-based-kasan-enable-fix
+++ a/mm/kmemleak.c
@@ -384,10 +384,10 @@ static struct kmemleak_object *lookup_ob

unsigned long untagged_ptr = (unsigned long)kasan_reset_tag((void *)ptr);

while (rb) {

- struct kmemleak_object *object =
- rb_entry(rb, struct kmemleak_object, rb_node);
+ struct kmemleak_object *object;
unsigned long untagged_objp;

+ object = rb_entry(rb, struct kmemleak_object, rb_node);

untagged_objp = (unsigned long)kasan_reset_tag((void *)object->pointer);

if (untagged_ptr < untagged_objp)
_