Good afternoon,
I'm looking for a little expert help with a out-of-bounds issue I've
been working on. Please let me know if my present understanding does
not quite match reality.
- Report
The following highly indeterministic kernel panic was reported to occur every
few hundred boots:
Kernel panic - not syncing: Fatal exception
RIP: 0010:alloc_ucounts+0x68/0x280
RSP: 0018:ffffb53ac13dfe98 EFLAGS: 00010006
RAX: 0000000000000000 RBX: 000000015b804063 RCX: ffff9bb60d5aa500
RDX: 0000000000000001 RSI: 00000000000003fb RDI: 0000000000000001
RBP: ffffb53ac13dfec0 R08: 0000000000000000 R09: ffffd53abf600000
R10: ffff9bb60b4bdd80 R11: ffffffffbcde7bb0 R12: ffffffffbf04e710
R13: ffffffffbf405160 R14: 00000000000003fb R15: ffffffffbf1afc60
FS: 00007f5be44d5000(0000) GS:ffff9bb6b9cc0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000015b80407b CR3: 00000001038ea000 CR4: 00000000000406a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__sys_setuid+0x1a0/0x290
__x64_sys_setuid+0xc/0x10
do_syscall_64+0x43/0x90
? asm_exc_page_fault+0x8/0x30
entry_SYSCALL_64_after_hwframe+0x44/0xae
- Problem
The issue was eventually tracked down to the attempted dereference of a value
located in a corrupted hash table. ucounts_hashtable is an array of 1024
struct hlists. Each element is the head of its own linked list where previous
ucount allocations are stored. The [20]th element of ucounts_hashtable was
being consistently trashed on each and every boot. However the indeterminism
comes from it being accessed only every few hundred boots.
The issue disappeared, or was at least unidentifiable when !(LTO=full) or when
memory base randomisation (a.k.a. KASLR) was disabled, rending GDB all but
impossible to use effectively.
The cause of the corruption was uncovered using a verity of different debugging
techniques and was eventually tracked down to page table manipulation in early
architecture setup.
The following line in arch/x86/realmode/init.c [0] allocates a variable, just 8
Bytes in size, to "hold the pgd entry used on booting additional CPUs":
pgd_t trampoline_pgd_entry;
The address of that variable is then passed from init_trampoline_kaslr() via a
call to set_pgd() [1] to have a value (not too important here) assigned to it.
Numerous abstractions take place, eventually leading to native_set_p4d(), an
inline function [2] contained in arch/x86/include/asm/pgtable_64.h.
From here, intentionally or otherwise, a call to pti_set_user_pgtbl() is made.
This is where the out-of-bounds write eventually occurs. It is not known (by
me) why this function is called. The returned result is subsequently used as a
value to write using the WRITE_ONCE macro. Perhaps the premature write is not
intended. This is what I hope to find out.
A little way down in pti_set_user_pgtbl() [3] the following line occurs:
kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd
The kernel_to_user_pgdp() part takes the address of pgdp (a.k.a.
trampoline_pgd_entry) and ends up flipping the 12th bit, essentially adding 4k
(0x1000) to the address. Then the part at the end assigns our value (still not
important here) to it. However, if we remember that only 8 Bytes was allocated
(globally) for trampoline_pgd_entry, then means we just stored the value into
the outlands (what we now know to be allocated to another global storage user
ucounts_hashtable).
- Ask
Hopefully I haven't messed anything up in the explanation here. Please let me
know if this is the case. I'm keen to understand what the intention was and
what we might do to fix it, if of course this hasn't already been remedied
somewhere.
As ever, any help / guidance would be gratefully received.
Kind regards,
Lee
[0] https://elixir.bootlin.com/linux/latest/source/arch/x86/realmode/init.c#L18
[1] https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/kaslr.c#L178
[2] https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/pgtable_64.h#L142
[3] https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/pti.c#L142
--
Lee Jones [李琼斯]
On Wed, 14 Jun 2023, Lee Jones wrote:
> Thanks for chiming in Dave. I hoped you would.
>
> On Wed, 14 Jun 2023, Dave Hansen wrote:
>
> > On 6/14/23 07:37, Lee Jones wrote:
> > > Still unsure how we (the kernel) can/should write to an area of memory
> > > that does not belong to it. Should we allocate enough memory
> > > (2*PAGE_SIZE? rather than 8-Bytes) for trampoline_pgd_entry to consume
> > > in a more sane way?
> >
> > No.
> >
> > I think this:
> >
> > set_pgd(&trampoline_pgd_entry,
> > __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> >
> > is bogus-ish. set_pgd() wants to operate on a pgd_t inside a pgd
> > *PAGE*. But it's just being pointed at a single _entry_. The address
> > of 'trampoline_pgd_entry' in your case also just (unfortunately)
> > happens to pass the:
> >
> > __pti_set_user_pgtbl -> pgdp_maps_userspace()
> >
> > test. I _think_ we want these to just be something like:
> >
> > trampoline_pgd_entry = __pgd(_KERNPG_TABLE |
> > __pa(p4d_page_tramp);
> >
> > That'll keep us away from all of the set_pgd()-induced nastiness.
>
> Okay. Is this what you're suggesting?
>
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c v
> index d336bb0cb38b..803595c7dcc8 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -176,7 +176,7 @@ void __meminit init_trampoline_kaslr(void)
> set_pgd(&trampoline_pgd_entry,
> __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> } else {
> - set_pgd(&trampoline_pgd_entry,
> - __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> + trampoline_pgd_entry =
> + __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp);
Note the change of *.page_tramp here.
s/pud/p4d/
I'm assuming that too was intentional?
> }
> }
>
> If so, I'll take it for a spin right now.
--
Lee Jones [李琼斯]
Thanks for chiming in Dave. I hoped you would.
On Wed, 14 Jun 2023, Dave Hansen wrote:
> On 6/14/23 07:37, Lee Jones wrote:
> > Still unsure how we (the kernel) can/should write to an area of memory
> > that does not belong to it. Should we allocate enough memory
> > (2*PAGE_SIZE? rather than 8-Bytes) for trampoline_pgd_entry to consume
> > in a more sane way?
>
> No.
>
> I think this:
>
> set_pgd(&trampoline_pgd_entry,
> __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
>
> is bogus-ish. set_pgd() wants to operate on a pgd_t inside a pgd
> *PAGE*. But it's just being pointed at a single _entry_. The address
> of 'trampoline_pgd_entry' in your case also just (unfortunately)
> happens to pass the:
>
> __pti_set_user_pgtbl -> pgdp_maps_userspace()
>
> test. I _think_ we want these to just be something like:
>
> trampoline_pgd_entry = __pgd(_KERNPG_TABLE |
> __pa(p4d_page_tramp);
>
> That'll keep us away from all of the set_pgd()-induced nastiness.
Okay. Is this what you're suggesting?
diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c v
index d336bb0cb38b..803595c7dcc8 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -176,7 +176,7 @@ void __meminit init_trampoline_kaslr(void)
set_pgd(&trampoline_pgd_entry,
__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
} else {
- set_pgd(&trampoline_pgd_entry,
- __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
+ trampoline_pgd_entry =
+ __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp);
}
}
If so, I'll take it for a spin right now.
--
Lee Jones [李琼斯]
On 6/14/23 08:26, Lee Jones wrote:
> On Wed, 14 Jun 2023, Lee Jones wrote:
>
>> On Wed, 14 Jun 2023, Lee Jones wrote:
>>
>>> Thanks for chiming in Dave. I hoped you would.
>>>
>>> On Wed, 14 Jun 2023, Dave Hansen wrote:
>>>
>>>> On 6/14/23 07:37, Lee Jones wrote:
>>>>> Still unsure how we (the kernel) can/should write to an area of memory
>>>>> that does not belong to it. Should we allocate enough memory
>>>>> (2*PAGE_SIZE? rather than 8-Bytes) for trampoline_pgd_entry to consume
>>>>> in a more sane way?
>>>>
>>>> No.
>>>>
>>>> I think this:
>>>>
>>>> set_pgd(&trampoline_pgd_entry,
>>>> __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
>>>>
>>>> is bogus-ish. set_pgd() wants to operate on a pgd_t inside a pgd
>>>> *PAGE*. But it's just being pointed at a single _entry_. The address
>>>> of 'trampoline_pgd_entry' in your case also just (unfortunately)
>>>> happens to pass the:
>>>>
>>>> __pti_set_user_pgtbl -> pgdp_maps_userspace()
>>>>
>>>> test. I _think_ we want these to just be something like:
>>>>
>>>> trampoline_pgd_entry = __pgd(_KERNPG_TABLE |
>>>> __pa(p4d_page_tramp);
>>>>
>>>> That'll keep us away from all of the set_pgd()-induced nastiness.
>>>
>>> Okay. Is this what you're suggesting?
>>>
>>> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c v
>>> index d336bb0cb38b..803595c7dcc8 100644
>>> --- a/arch/x86/mm/kaslr.c
>>> +++ b/arch/x86/mm/kaslr.c
>>> @@ -176,7 +176,7 @@ void __meminit init_trampoline_kaslr(void)
>>> set_pgd(&trampoline_pgd_entry,
>>> __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
>>> } else {
>>> - set_pgd(&trampoline_pgd_entry,
>>> - __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
>>> + trampoline_pgd_entry =
>>> + __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp);
>>
>> Note the change of *.page_tramp here.
>>
>> s/pud/p4d/
>>
>> I'm assuming that too was intentional?
>
> Never mind. I can see that p4d_page_tramp is local to the if() segment.
>
> While we're at it, does the if() segment look correct to you:
>
> if (pgtable_l5_enabled()) {
> p4d_page_tramp = alloc_low_page();
>
> p4d_tramp = p4d_page_tramp + p4d_index(paddr);
>
> set_p4d(p4d_tramp,
> __p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
>
> set_pgd(&trampoline_pgd_entry,
> __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> } else {
> trampoline_pgd_entry =
> __pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
> }
>
> - pud_page_tramp is being passed to set_p4d()
> - p4d_page_tramp is being passed to set_pgd()
>
> Should those be the other way around, or am I missing the point?
You're missing the point. :)
PGDs are always set up to point to the physical address of the thing at
one lower level than them. A page is allocated for that level when
5-level paging is in play. No page is needed when it is not in play.
The pattern is _almost_ always
pgd = ... __pa(p4d);
In other words, point the PGD at the physical address of a p4d. But
things get funky on systems without p4ds, thus the special casing here.
Does the (completely untested) attached patch fix your problem?
On Wed, 14 Jun 2023, Lee Jones wrote:
> On Wed, 14 Jun 2023, Lee Jones wrote:
>
> > Thanks for chiming in Dave. I hoped you would.
> >
> > On Wed, 14 Jun 2023, Dave Hansen wrote:
> >
> > > On 6/14/23 07:37, Lee Jones wrote:
> > > > Still unsure how we (the kernel) can/should write to an area of memory
> > > > that does not belong to it. Should we allocate enough memory
> > > > (2*PAGE_SIZE? rather than 8-Bytes) for trampoline_pgd_entry to consume
> > > > in a more sane way?
> > >
> > > No.
> > >
> > > I think this:
> > >
> > > set_pgd(&trampoline_pgd_entry,
> > > __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> > >
> > > is bogus-ish. set_pgd() wants to operate on a pgd_t inside a pgd
> > > *PAGE*. But it's just being pointed at a single _entry_. The address
> > > of 'trampoline_pgd_entry' in your case also just (unfortunately)
> > > happens to pass the:
> > >
> > > __pti_set_user_pgtbl -> pgdp_maps_userspace()
> > >
> > > test. I _think_ we want these to just be something like:
> > >
> > > trampoline_pgd_entry = __pgd(_KERNPG_TABLE |
> > > __pa(p4d_page_tramp);
> > >
> > > That'll keep us away from all of the set_pgd()-induced nastiness.
> >
> > Okay. Is this what you're suggesting?
> >
> > diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c v
> > index d336bb0cb38b..803595c7dcc8 100644
> > --- a/arch/x86/mm/kaslr.c
> > +++ b/arch/x86/mm/kaslr.c
> > @@ -176,7 +176,7 @@ void __meminit init_trampoline_kaslr(void)
> > set_pgd(&trampoline_pgd_entry,
> > __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> > } else {
> > - set_pgd(&trampoline_pgd_entry,
> > - __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> > + trampoline_pgd_entry =
> > + __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp);
>
> Note the change of *.page_tramp here.
>
> s/pud/p4d/
>
> I'm assuming that too was intentional?
Never mind. I can see that p4d_page_tramp is local to the if() segment.
While we're at it, does the if() segment look correct to you:
if (pgtable_l5_enabled()) {
p4d_page_tramp = alloc_low_page();
p4d_tramp = p4d_page_tramp + p4d_index(paddr);
set_p4d(p4d_tramp,
__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
set_pgd(&trampoline_pgd_entry,
__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
} else {
trampoline_pgd_entry =
__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
}
- pud_page_tramp is being passed to set_p4d()
- p4d_page_tramp is being passed to set_pgd()
Should those be the other way around, or am I missing the point?
--
Lee Jones [李琼斯]
On Wed, 14 Jun 2023, Dave Hansen wrote:
> On 6/14/23 08:26, Lee Jones wrote:
> > On Wed, 14 Jun 2023, Lee Jones wrote:
> >
> >> On Wed, 14 Jun 2023, Lee Jones wrote:
> >>
> >>> Thanks for chiming in Dave. I hoped you would.
> >>>
> >>> On Wed, 14 Jun 2023, Dave Hansen wrote:
> >>>
> >>>> On 6/14/23 07:37, Lee Jones wrote:
> >>>>> Still unsure how we (the kernel) can/should write to an area of memory
> >>>>> that does not belong to it. Should we allocate enough memory
> >>>>> (2*PAGE_SIZE? rather than 8-Bytes) for trampoline_pgd_entry to consume
> >>>>> in a more sane way?
> >>>>
> >>>> No.
> >>>>
> >>>> I think this:
> >>>>
> >>>> set_pgd(&trampoline_pgd_entry,
> >>>> __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> >>>>
> >>>> is bogus-ish. set_pgd() wants to operate on a pgd_t inside a pgd
> >>>> *PAGE*. But it's just being pointed at a single _entry_. The address
> >>>> of 'trampoline_pgd_entry' in your case also just (unfortunately)
> >>>> happens to pass the:
> >>>>
> >>>> __pti_set_user_pgtbl -> pgdp_maps_userspace()
> >>>>
> >>>> test. I _think_ we want these to just be something like:
> >>>>
> >>>> trampoline_pgd_entry = __pgd(_KERNPG_TABLE |
> >>>> __pa(p4d_page_tramp);
> >>>>
> >>>> That'll keep us away from all of the set_pgd()-induced nastiness.
> >>>
> >>> Okay. Is this what you're suggesting?
> >>>
> >>> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c v
> >>> index d336bb0cb38b..803595c7dcc8 100644
> >>> --- a/arch/x86/mm/kaslr.c
> >>> +++ b/arch/x86/mm/kaslr.c
> >>> @@ -176,7 +176,7 @@ void __meminit init_trampoline_kaslr(void)
> >>> set_pgd(&trampoline_pgd_entry,
> >>> __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> >>> } else {
> >>> - set_pgd(&trampoline_pgd_entry,
> >>> - __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> >>> + trampoline_pgd_entry =
> >>> + __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp);
> >>
> >> Note the change of *.page_tramp here.
> >>
> >> s/pud/p4d/
> >>
> >> I'm assuming that too was intentional?
> >
> > Never mind. I can see that p4d_page_tramp is local to the if() segment.
> >
> > While we're at it, does the if() segment look correct to you:
> >
> > if (pgtable_l5_enabled()) {
> > p4d_page_tramp = alloc_low_page();
> >
> > p4d_tramp = p4d_page_tramp + p4d_index(paddr);
> >
> > set_p4d(p4d_tramp,
> > __p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
> >
> > set_pgd(&trampoline_pgd_entry,
> > __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> > } else {
> > trampoline_pgd_entry =
> > __pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
> > }
> >
> > - pud_page_tramp is being passed to set_p4d()
> > - p4d_page_tramp is being passed to set_pgd()
> >
> > Should those be the other way around, or am I missing the point?
>
> You're missing the point. :)
Super, thanks for the explanation.
> PGDs are always set up to point to the physical address of the thing at
> one lower level than them. A page is allocated for that level when
> 5-level paging is in play. No page is needed when it is not in play.
>
> The pattern is _almost_ always
>
> pgd = ... __pa(p4d);
>
> In other words, point the PGD at the physical address of a p4d. But
> things get funky on systems without p4ds, thus the special casing here.
>
> Does the (completely untested) attached patch fix your problem?
I just submitted a (tested) patch.
It doesn't cover the if() segment though. I'll do so and resubmit.
> ---
>
> b/arch/x86/mm/kaslr.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff -puN arch/x86/mm/kaslr.c~trampoline_pgd_entry arch/x86/mm/kaslr.c
> --- a/arch/x86/mm/kaslr.c~trampoline_pgd_entry 2023-06-14 08:54:08.685554094 -0700
> +++ b/arch/x86/mm/kaslr.c 2023-06-14 08:55:36.077089793 -0700
> @@ -172,10 +172,10 @@ void __meminit init_trampoline_kaslr(voi
> set_p4d(p4d_tramp,
> __p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
>
> - set_pgd(&trampoline_pgd_entry,
> - __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> + trampoline_pgd_entry =
> + __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
> } else {
> - set_pgd(&trampoline_pgd_entry,
> - __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> + trampoline_pgd_entry =
> + __pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
> }
> }
> _
--
Lee Jones [李琼斯]