On Sat, Mar 26, 2022 at 10:19 PM Muchun Song <[email protected]> wrote:
>
> The objcg is not cleared and put for kfence object when it is freed, which
> could lead to memory leak for struct obj_cgroup and wrong statistics of
> NR_SLAB_RECLAIMABLE_B or NR_SLAB_UNRECLAIMABLE_B. Since the last freed
> object's objcg is not cleared, mem_cgroup_from_obj() could return the wrong
> memcg when this kfence object, which is not charged to any objcgs, is
> reallocated to other users. A real word issue [1] is caused by this bug.
Good that this looks sorted out.
Patch 2/2 seems to still be up in the air. The patch not only causes
build errors, but it looks really very odd to me.
In particular, you do that loop with
__SetPageSlab(&pages[i]);
in kfence_init_pool(), but that is *not* where you set the
MEMCG_DATA_OBJCGS, and instead do that virt_to_slab(addr) dance later.
That looks very odd to me. I think the two should go hand-in-hand,
since that __SetPageSlab() really is what makes it a slab thing, and I
think it should go together with setting the slab state correctly.
Finally, is there a syzbot report for that second problem?
Anyway, should I apply this PATCH 1/2 now directly as the solution for
the dentry issue, or should I wait for that second patch? They seem to
be related only indirectly, in that the problems were both introduced
by the same commit.
Linus
On Mon, Mar 28, 2022 at 5:08 AM Linus Torvalds
<[email protected]> wrote:
>
> On Sat, Mar 26, 2022 at 10:19 PM Muchun Song <[email protected]> wrote:
> >
> > The objcg is not cleared and put for kfence object when it is freed, which
> > could lead to memory leak for struct obj_cgroup and wrong statistics of
> > NR_SLAB_RECLAIMABLE_B or NR_SLAB_UNRECLAIMABLE_B. Since the last freed
> > object's objcg is not cleared, mem_cgroup_from_obj() could return the wrong
> > memcg when this kfence object, which is not charged to any objcgs, is
> > reallocated to other users. A real word issue [1] is caused by this bug.
>
> Good that this looks sorted out.
>
> Patch 2/2 seems to still be up in the air. The patch not only causes
> build errors, but it looks really very odd to me.
>
> In particular, you do that loop with
>
> __SetPageSlab(&pages[i]);
>
> in kfence_init_pool(), but that is *not* where you set the
> MEMCG_DATA_OBJCGS, and instead do that virt_to_slab(addr) dance later.
>
> That looks very odd to me. I think the two should go hand-in-hand,
> since that __SetPageSlab() really is what makes it a slab thing, and I
> think it should go together with setting the slab state correctly.
Right. It is a little odd. I'll improve it in the next version.
>
> Finally, is there a syzbot report for that second problem?
No. The second bug does not trigger any oops, so it is hard to be seen.
It is just my code review.
>
> Anyway, should I apply this PATCH 1/2 now directly as the solution for
> the dentry issue, or should I wait for that second patch? They seem to
> be related only indirectly, in that the problems were both introduced
> by the same commit.
>
I think you could apply PATCH 1/2 now. PATCH 2/2 is another issue not
related to dentry issue.
Thanks.