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

[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-Ying.Lee@xxxxxxxxxxxx> 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);

> 
> > > 
> > > 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-Ying.Lee@xxxxxxxxxxxx>
> > > ---
> > >  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
> > > 
> > 
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