LinuxLists.cc - [PATCH] kasan: Add explicit preconditions to kasan

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
<[email protected]> wrote:
>
> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
> the address passed as a parameter.
>
> Add a comment to make sure that the preconditions to the function are
> explicitly clarified.
>
> Note: An invalid address (e.g. NULL pointer address) passed to the
> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
>
> Cc: Andrey Ryabinin <[email protected]>
> Cc: Alexander Potapenko <[email protected]>
> Cc: Dmitry Vyukov <[email protected]>
> Cc: Leon Romanovsky <[email protected]>
> Cc: Andrey Konovalov <[email protected]>
> Signed-off-by: Vincenzo Frascino <[email protected]>
> ---
> mm/kasan/report.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index c0fb21797550..2485b585004d 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
> end_report(&flags);
> }
>
> +/**
> + * kasan_report - report kasan fault details
> + * @addr: valid address of the allocation where the tag fault was detected
> + * @size: size of the allocation where the tag fault was detected
> + * @is_write: the instruction that caused the fault was a read or write?
> + * @ip: pointer to the instruction that cause the fault
> + *
> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
> + * the address to access the tags, hence it must be valid at this point in
> + * order to not cause a kernel panic.
> + */

It doesn't dereference the address, it just checks the tags, right?

Ideally, kasan_report() should survive that with HW_TAGS like with the
other modes. The reason it doesn't is probably because of a blank
addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
guess we should somehow check that the memory comes from page_alloc or
kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
instruction to check whether the memory has tags?

2021-01-19 19:13:55

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

Hi Andrey,

On 1/19/21 6:27 PM, Andrey Konovalov wrote:
> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
> <[email protected]> wrote:
>>
>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
>> the address passed as a parameter.
>>
>> Add a comment to make sure that the preconditions to the function are
>> explicitly clarified.
>>
>> Note: An invalid address (e.g. NULL pointer address) passed to the
>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
>>
>> Cc: Andrey Ryabinin <[email protected]>
>> Cc: Alexander Potapenko <[email protected]>
>> Cc: Dmitry Vyukov <[email protected]>
>> Cc: Leon Romanovsky <[email protected]>
>> Cc: Andrey Konovalov <[email protected]>
>> Signed-off-by: Vincenzo Frascino <[email protected]>
>> ---
>> mm/kasan/report.c | 11 +++++++++++
>> 1 file changed, 11 insertions(+)
>>
>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
>> index c0fb21797550..2485b585004d 100644
>> --- a/mm/kasan/report.c
>> +++ b/mm/kasan/report.c
>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
>> end_report(&flags);
>> }
>>
>> +/**
>> + * kasan_report - report kasan fault details
>> + * @addr: valid address of the allocation where the tag fault was detected
>> + * @size: size of the allocation where the tag fault was detected
>> + * @is_write: the instruction that caused the fault was a read or write?
>> + * @ip: pointer to the instruction that cause the fault
>> + *
>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
>> + * the address to access the tags, hence it must be valid at this point in
>> + * order to not cause a kernel panic.
>> + */
>
> It doesn't dereference the address, it just checks the tags, right?
>

This is correct, just realized that the use of "dereference" here is misleading.

> Ideally, kasan_report() should survive that with HW_TAGS like with the
> other modes. The reason it doesn't is probably because of a blank
> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
> guess we should somehow check that the memory comes from page_alloc or
> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
> instruction to check whether the memory has tags?
>

I agree, looking a second time at the code the problem comes from
addr_has_metadata():

...

[ 18.127273] BUG: KASAN: invalid-access in 0x0
[ 18.128604] Read at addr 0000000000000000 by task swapper/0/1
[ 18.130311] Unable to handle kernel NULL pointer dereference at virtual
address 0000000000000000
[ 18.131291] Mem abort info:
[ 18.131696] ESR = 0x96000004
[ 18.132169] EC = 0x25: DABT (current EL), IL = 32 bits
[ 18.132953] SET = 0, FnV = 0
[ 18.133433] EA = 0, S1PTW = 0
[ 18.133907] Data abort info:
[ 18.134308] ISV = 0, ISS = 0x00000004
[ 18.134883] CM = 0, WnR = 0
[ 18.135436] [0000000000000000] user address but active_mm is swapper
[ 18.136372] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[ 18.137280] Modules linked in:
[ 18.138182] CPU: 2 PID: 1 Comm: swapper/0 Not tainted
5.11.0-rc4-00007-g86cba71f117-dirty #2
[ 18.139275] Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
[ 18.140342] pstate: 60400085 (nZCv daIf +PAN -UAO -TCO BTYPE=--)
[ 18.141324] pc : mte_get_mem_tag+0x24/0x40
[ 18.142487] lr : print_tags+0x1c/0x40
[ 18.143095] sp : ffff80001004bcf0
[ 18.143570] x29: ffff80001004bcf0 x28: 0000000000000000
[ 18.144526] x27: ffffd042f0bf04e0 x26: ffffd042f0ca1068
[ 18.145369] x25: ffffd042f0bdde58 x24: ffffd042f1458000
[ 18.146209] x23: 0000000000000000 x22: 0000000000000000
[ 18.147047] x21: 0000000000000000 x20: 0000000000000000
[ 18.147928] x19: 0000000000000000 x18: ffffffffffffffff
[ 18.148928] x17: 000000000000000e x16: 0000000000000001
[ 18.149837] x15: ffff80009004ba17 x14: 0000000000000006
[ 18.150774] x13: ffffd042f11b27e0 x12: 0000000000000399
[ 18.151653] x11: 0000000000000133 x10: ffffd042f11b27e0
[ 18.152544] x9 : ffffd042f11b27e0 x8 : 00000000ffffefff
[ 18.153443] x7 : ffffd042f120a7e0 x6 : ffffd042f120a7e0
[ 18.154272] x5 : 000000000000bff4 x4 : 0000000000000000
[ 18.155096] x3 : 0000000000000000 x2 : 0000000000000000
[ 18.155958] x1 : 0000000000000000 x0 : 0000000000000000
[ 18.157145] Call trace:
[ 18.157615] mte_get_mem_tag+0x24/0x40
[ 18.158258] kasan_report+0xec/0x1b0

...

I noticed it differently but you can easily reproduce it calling
kasan_report(0,0,0,0); from somewhere.

I will send a patch tomorrow that checks if the memory comes from page_alloc or
kmalloc. Not sure what you mean for "instruction to check whether the memory has
tags".

Thanks!

--
Regards,
Vincenzo

2021-01-19 19:16:38

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On 1/19/21 6:52 PM, Catalin Marinas wrote:
> On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
>> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
>> <[email protected]> wrote:
>>>
>>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
>>> the address passed as a parameter.
>>>
>>> Add a comment to make sure that the preconditions to the function are
>>> explicitly clarified.
>>>
>>> Note: An invalid address (e.g. NULL pointer address) passed to the
>>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
>>>
>>> Cc: Andrey Ryabinin <[email protected]>
>>> Cc: Alexander Potapenko <[email protected]>
>>> Cc: Dmitry Vyukov <[email protected]>
>>> Cc: Leon Romanovsky <[email protected]>
>>> Cc: Andrey Konovalov <[email protected]>
>>> Signed-off-by: Vincenzo Frascino <[email protected]>
>>> ---
>>> mm/kasan/report.c | 11 +++++++++++
>>> 1 file changed, 11 insertions(+)
>>>
>>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
>>> index c0fb21797550..2485b585004d 100644
>>> --- a/mm/kasan/report.c
>>> +++ b/mm/kasan/report.c
>>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
>>> end_report(&flags);
>>> }
>>>
>>> +/**
>>> + * kasan_report - report kasan fault details
>>> + * @addr: valid address of the allocation where the tag fault was detected
>>> + * @size: size of the allocation where the tag fault was detected
>>> + * @is_write: the instruction that caused the fault was a read or write?
>>> + * @ip: pointer to the instruction that cause the fault
>>> + *
>>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
>>> + * the address to access the tags, hence it must be valid at this point in
>>> + * order to not cause a kernel panic.
>>> + */
>>
>> It doesn't dereference the address, it just checks the tags, right?
>>
>> Ideally, kasan_report() should survive that with HW_TAGS like with the
>> other modes. The reason it doesn't is probably because of a blank
>> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
>> guess we should somehow check that the memory comes from page_alloc or
>> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
>> instruction to check whether the memory has tags?
>
> There isn't an architected way to probe whether a memory location has a
> VA->PA mapping. The tags are addressed by PA but you can't reach them if
> you get a page fault on the VA. So we either document the kasan_report()
> preconditions or, as you suggest, update addr_has_metadata() for the
> HW_TAGS case. Something like:
>
> return is_vmalloc_addr(virt) || virt_addr_valid(virt));
>

Or we could have both ;)

--
Regards,
Vincenzo

2021-01-19 19:17:13

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On 1/19/21 7:02 PM, Catalin Marinas wrote:
> On Tue, Jan 19, 2021 at 07:00:57PM +0000, Vincenzo Frascino wrote:
>> On 1/19/21 6:52 PM, Catalin Marinas wrote:
>>> On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
>>>> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
>>>> <[email protected]> wrote:
>>>>>
>>>>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
>>>>> the address passed as a parameter.
>>>>>
>>>>> Add a comment to make sure that the preconditions to the function are
>>>>> explicitly clarified.
>>>>>
>>>>> Note: An invalid address (e.g. NULL pointer address) passed to the
>>>>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
>>>>>
>>>>> Cc: Andrey Ryabinin <[email protected]>
>>>>> Cc: Alexander Potapenko <[email protected]>
>>>>> Cc: Dmitry Vyukov <[email protected]>
>>>>> Cc: Leon Romanovsky <[email protected]>
>>>>> Cc: Andrey Konovalov <[email protected]>
>>>>> Signed-off-by: Vincenzo Frascino <[email protected]>
>>>>> ---
>>>>> mm/kasan/report.c | 11 +++++++++++
>>>>> 1 file changed, 11 insertions(+)
>>>>>
>>>>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
>>>>> index c0fb21797550..2485b585004d 100644
>>>>> --- a/mm/kasan/report.c
>>>>> +++ b/mm/kasan/report.c
>>>>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
>>>>> end_report(&flags);
>>>>> }
>>>>>
>>>>> +/**
>>>>> + * kasan_report - report kasan fault details
>>>>> + * @addr: valid address of the allocation where the tag fault was detected
>>>>> + * @size: size of the allocation where the tag fault was detected
>>>>> + * @is_write: the instruction that caused the fault was a read or write?
>>>>> + * @ip: pointer to the instruction that cause the fault
>>>>> + *
>>>>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
>>>>> + * the address to access the tags, hence it must be valid at this point in
>>>>> + * order to not cause a kernel panic.
>>>>> + */
>>>>
>>>> It doesn't dereference the address, it just checks the tags, right?
>>>>
>>>> Ideally, kasan_report() should survive that with HW_TAGS like with the
>>>> other modes. The reason it doesn't is probably because of a blank
>>>> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
>>>> guess we should somehow check that the memory comes from page_alloc or
>>>> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
>>>> instruction to check whether the memory has tags?
>>>
>>> There isn't an architected way to probe whether a memory location has a
>>> VA->PA mapping. The tags are addressed by PA but you can't reach them if
>>> you get a page fault on the VA. So we either document the kasan_report()
>>> preconditions or, as you suggest, update addr_has_metadata() for the
>>> HW_TAGS case. Something like:
>>>
>>> return is_vmalloc_addr(virt) || virt_addr_valid(virt));
>>
>> Or we could have both ;)
>
> True. Documentation doesn't hurt (well, only when it's wrong ;)).
>

Testing the patch now, I will send it in half an hour.

--
Regards,
Vincenzo

2021-01-19 19:17:59

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Tue, Jan 19, 2021 at 07:00:57PM +0000, Vincenzo Frascino wrote:
> On 1/19/21 6:52 PM, Catalin Marinas wrote:
> > On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
> >> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
> >> <[email protected]> wrote:
> >>>
> >>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
> >>> the address passed as a parameter.
> >>>
> >>> Add a comment to make sure that the preconditions to the function are
> >>> explicitly clarified.
> >>>
> >>> Note: An invalid address (e.g. NULL pointer address) passed to the
> >>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
> >>>
> >>> Cc: Andrey Ryabinin <[email protected]>
> >>> Cc: Alexander Potapenko <[email protected]>
> >>> Cc: Dmitry Vyukov <[email protected]>
> >>> Cc: Leon Romanovsky <[email protected]>
> >>> Cc: Andrey Konovalov <[email protected]>
> >>> Signed-off-by: Vincenzo Frascino <[email protected]>
> >>> ---
> >>> mm/kasan/report.c | 11 +++++++++++
> >>> 1 file changed, 11 insertions(+)
> >>>
> >>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> >>> index c0fb21797550..2485b585004d 100644
> >>> --- a/mm/kasan/report.c
> >>> +++ b/mm/kasan/report.c
> >>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
> >>> end_report(&flags);
> >>> }
> >>>
> >>> +/**
> >>> + * kasan_report - report kasan fault details
> >>> + * @addr: valid address of the allocation where the tag fault was detected
> >>> + * @size: size of the allocation where the tag fault was detected
> >>> + * @is_write: the instruction that caused the fault was a read or write?
> >>> + * @ip: pointer to the instruction that cause the fault
> >>> + *
> >>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
> >>> + * the address to access the tags, hence it must be valid at this point in
> >>> + * order to not cause a kernel panic.
> >>> + */
> >>
> >> It doesn't dereference the address, it just checks the tags, right?
> >>
> >> Ideally, kasan_report() should survive that with HW_TAGS like with the
> >> other modes. The reason it doesn't is probably because of a blank
> >> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
> >> guess we should somehow check that the memory comes from page_alloc or
> >> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
> >> instruction to check whether the memory has tags?
> >
> > There isn't an architected way to probe whether a memory location has a
> > VA->PA mapping. The tags are addressed by PA but you can't reach them if
> > you get a page fault on the VA. So we either document the kasan_report()
> > preconditions or, as you suggest, update addr_has_metadata() for the
> > HW_TAGS case. Something like:
> >
> > return is_vmalloc_addr(virt) || virt_addr_valid(virt));
>
> Or we could have both ;)

True. Documentation doesn't hurt (well, only when it's wrong ;)).

--
Catalin

2021-01-19 19:38:00

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Tue, Jan 19, 2021 at 7:57 PM Vincenzo Frascino
<[email protected]> wrote:
>
>
>
> On 1/19/21 6:52 PM, Catalin Marinas wrote:
> > On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
> >> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
> >> <[email protected]> wrote:
> >>>
> >>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
> >>> the address passed as a parameter.
> >>>
> >>> Add a comment to make sure that the preconditions to the function are
> >>> explicitly clarified.
> >>>
> >>> Note: An invalid address (e.g. NULL pointer address) passed to the
> >>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
> >>>
> >>> Cc: Andrey Ryabinin <[email protected]>
> >>> Cc: Alexander Potapenko <[email protected]>
> >>> Cc: Dmitry Vyukov <[email protected]>
> >>> Cc: Leon Romanovsky <[email protected]>
> >>> Cc: Andrey Konovalov <[email protected]>
> >>> Signed-off-by: Vincenzo Frascino <[email protected]>
> >>> ---
> >>> mm/kasan/report.c | 11 +++++++++++
> >>> 1 file changed, 11 insertions(+)
> >>>
> >>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> >>> index c0fb21797550..2485b585004d 100644
> >>> --- a/mm/kasan/report.c
> >>> +++ b/mm/kasan/report.c
> >>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
> >>> end_report(&flags);
> >>> }
> >>>
> >>> +/**
> >>> + * kasan_report - report kasan fault details
> >>> + * @addr: valid address of the allocation where the tag fault was detected
> >>> + * @size: size of the allocation where the tag fault was detected
> >>> + * @is_write: the instruction that caused the fault was a read or write?
> >>> + * @ip: pointer to the instruction that cause the fault
> >>> + *
> >>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
> >>> + * the address to access the tags, hence it must be valid at this point in
> >>> + * order to not cause a kernel panic.
> >>> + */
> >>
> >> It doesn't dereference the address, it just checks the tags, right?
> >>
> >> Ideally, kasan_report() should survive that with HW_TAGS like with the
> >> other modes. The reason it doesn't is probably because of a blank
> >> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
> >> guess we should somehow check that the memory comes from page_alloc or
> >> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
> >> instruction to check whether the memory has tags?
> >
> > There isn't an architected way to probe whether a memory location has a
> > VA->PA mapping. The tags are addressed by PA but you can't reach them if
> > you get a page fault on the VA. So we either document the kasan_report()
> > preconditions or, as you suggest, update addr_has_metadata() for the
> > HW_TAGS case. Something like:
> >
> > return is_vmalloc_addr(virt) || virt_addr_valid(virt));
> >
>
> Or we could have both ;)

Sounds good! Please also update the comment to avoid mentioning tag
faults. kasan_report() is used for the generic KASAN mode as well, and
it doesn't use tags.

Thanks!

2021-01-19 20:23:49

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
> <[email protected]> wrote:
> >
> > With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
> > the address passed as a parameter.
> >
> > Add a comment to make sure that the preconditions to the function are
> > explicitly clarified.
> >
> > Note: An invalid address (e.g. NULL pointer address) passed to the
> > function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
> >
> > Cc: Andrey Ryabinin <[email protected]>
> > Cc: Alexander Potapenko <[email protected]>
> > Cc: Dmitry Vyukov <[email protected]>
> > Cc: Leon Romanovsky <[email protected]>
> > Cc: Andrey Konovalov <[email protected]>
> > Signed-off-by: Vincenzo Frascino <[email protected]>
> > ---
> > mm/kasan/report.c | 11 +++++++++++
> > 1 file changed, 11 insertions(+)
> >
> > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > index c0fb21797550..2485b585004d 100644
> > --- a/mm/kasan/report.c
> > +++ b/mm/kasan/report.c
> > @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
> > end_report(&flags);
> > }
> >
> > +/**
> > + * kasan_report - report kasan fault details
> > + * @addr: valid address of the allocation where the tag fault was detected
> > + * @size: size of the allocation where the tag fault was detected
> > + * @is_write: the instruction that caused the fault was a read or write?
> > + * @ip: pointer to the instruction that cause the fault
> > + *
> > + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
> > + * the address to access the tags, hence it must be valid at this point in
> > + * order to not cause a kernel panic.
> > + */
>
> It doesn't dereference the address, it just checks the tags, right?
>
> Ideally, kasan_report() should survive that with HW_TAGS like with the
> other modes. The reason it doesn't is probably because of a blank
> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
> guess we should somehow check that the memory comes from page_alloc or
> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
> instruction to check whether the memory has tags?

There isn't an architected way to probe whether a memory location has a
VA->PA mapping. The tags are addressed by PA but you can't reach them if
you get a page fault on the VA. So we either document the kasan_report()
preconditions or, as you suggest, update addr_has_metadata() for the
HW_TAGS case. Something like:

return is_vmalloc_addr(virt) || virt_addr_valid(virt));

--
Catalin

2021-01-19 20:36:52

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Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

2021-01-19 20:59:22

[permalink] [raw]

Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Tue, Jan 19, 2021 at 9:32 PM Vincenzo Frascino
<[email protected]> wrote:
>
> This seems not working on arm64 because according to virt_addr_valid 0 is a
> valid virtual address, in fact:
>
> __is_lm_address(0) == true && pfn_valid(virt_to_pfn(0)) == true.
>
> An option could be to make an exception for virtual address 0 in
> addr_has_metadata() something like:
>
> static inline bool addr_has_metadata(const void *addr)
> {
> if ((u64)addr == 0)
> return false;

This sounds good to me, but we need to check for < PAGE_SIZE or
something like that, right? There's some limit below which accesses
are considered null-ptr-derefs.

> return (is_vmalloc_addr(addr) || virt_addr_valid(addr));

Do we need is_vmalloc_addr()? As we don't yet have vmalloc support for HW_TAGS.

2021-01-20 16:10:10

[permalink] [raw]

Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Tue, Jan 19, 2021 at 08:35:49PM +0000, Vincenzo Frascino wrote:
> On 1/19/21 6:52 PM, Catalin Marinas wrote:
> > On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
> >> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
> >> <[email protected]> wrote:
> >>>
> >>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
> >>> the address passed as a parameter.
> >>>
> >>> Add a comment to make sure that the preconditions to the function are
> >>> explicitly clarified.
> >>>
> >>> Note: An invalid address (e.g. NULL pointer address) passed to the
> >>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
> >>>
> >>> Cc: Andrey Ryabinin <[email protected]>
> >>> Cc: Alexander Potapenko <[email protected]>
> >>> Cc: Dmitry Vyukov <[email protected]>
> >>> Cc: Leon Romanovsky <[email protected]>
> >>> Cc: Andrey Konovalov <[email protected]>
> >>> Signed-off-by: Vincenzo Frascino <[email protected]>
> >>> ---
> >>> mm/kasan/report.c | 11 +++++++++++
> >>> 1 file changed, 11 insertions(+)
> >>>
> >>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> >>> index c0fb21797550..2485b585004d 100644
> >>> --- a/mm/kasan/report.c
> >>> +++ b/mm/kasan/report.c
> >>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
> >>> end_report(&flags);
> >>> }
> >>>
> >>> +/**
> >>> + * kasan_report - report kasan fault details
> >>> + * @addr: valid address of the allocation where the tag fault was detected
> >>> + * @size: size of the allocation where the tag fault was detected
> >>> + * @is_write: the instruction that caused the fault was a read or write?
> >>> + * @ip: pointer to the instruction that cause the fault
> >>> + *
> >>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
> >>> + * the address to access the tags, hence it must be valid at this point in
> >>> + * order to not cause a kernel panic.
> >>> + */
> >>
> >> It doesn't dereference the address, it just checks the tags, right?
> >>
> >> Ideally, kasan_report() should survive that with HW_TAGS like with the
> >> other modes. The reason it doesn't is probably because of a blank
> >> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
> >> guess we should somehow check that the memory comes from page_alloc or
> >> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
> >> instruction to check whether the memory has tags?
> >
> > There isn't an architected way to probe whether a memory location has a
> > VA->PA mapping. The tags are addressed by PA but you can't reach them if
> > you get a page fault on the VA. So we either document the kasan_report()
> > preconditions or, as you suggest, update addr_has_metadata() for the
> > HW_TAGS case. Something like:
> >
> > return is_vmalloc_addr(virt) || virt_addr_valid(virt));
> >
>
> This seems not working on arm64 because according to virt_addr_valid 0 is a
> valid virtual address, in fact:
>
> __is_lm_address(0) == true && pfn_valid(virt_to_pfn(0)) == true.

Ah, so __is_lm_address(0) is true. Maybe we should improve this since
virt_to_pfn(0) doesn't make much sense.

> An option could be to make an exception for virtual address 0 in
> addr_has_metadata() something like:
>
> static inline bool addr_has_metadata(const void *addr)
> {
> if ((u64)addr == 0)
> return false;
>
> return (is_vmalloc_addr(addr) || virt_addr_valid(addr));
> }

As Andrey replied, passing a non-zero small value would still be
incorrectly detected as valid.

--
Catalin

2021-01-20 16:23:10

[permalink] [raw]

Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On 1/20/21 4:04 PM, Catalin Marinas wrote:
> On Tue, Jan 19, 2021 at 08:35:49PM +0000, Vincenzo Frascino wrote:
>> On 1/19/21 6:52 PM, Catalin Marinas wrote:
>>> On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
>>>> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
>>>> <[email protected]> wrote:
>>>>>
>>>>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
>>>>> the address passed as a parameter.
>>>>>
>>>>> Add a comment to make sure that the preconditions to the function are
>>>>> explicitly clarified.
>>>>>
>>>>> Note: An invalid address (e.g. NULL pointer address) passed to the
>>>>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
>>>>>
>>>>> Cc: Andrey Ryabinin <[email protected]>
>>>>> Cc: Alexander Potapenko <[email protected]>
>>>>> Cc: Dmitry Vyukov <[email protected]>
>>>>> Cc: Leon Romanovsky <[email protected]>
>>>>> Cc: Andrey Konovalov <[email protected]>
>>>>> Signed-off-by: Vincenzo Frascino <[email protected]>
>>>>> ---
>>>>> mm/kasan/report.c | 11 +++++++++++
>>>>> 1 file changed, 11 insertions(+)
>>>>>
>>>>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
>>>>> index c0fb21797550..2485b585004d 100644
>>>>> --- a/mm/kasan/report.c
>>>>> +++ b/mm/kasan/report.c
>>>>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
>>>>> end_report(&flags);
>>>>> }
>>>>>
>>>>> +/**
>>>>> + * kasan_report - report kasan fault details
>>>>> + * @addr: valid address of the allocation where the tag fault was detected
>>>>> + * @size: size of the allocation where the tag fault was detected
>>>>> + * @is_write: the instruction that caused the fault was a read or write?
>>>>> + * @ip: pointer to the instruction that cause the fault
>>>>> + *
>>>>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
>>>>> + * the address to access the tags, hence it must be valid at this point in
>>>>> + * order to not cause a kernel panic.
>>>>> + */
>>>>
>>>> It doesn't dereference the address, it just checks the tags, right?
>>>>
>>>> Ideally, kasan_report() should survive that with HW_TAGS like with the
>>>> other modes. The reason it doesn't is probably because of a blank
>>>> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
>>>> guess we should somehow check that the memory comes from page_alloc or
>>>> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
>>>> instruction to check whether the memory has tags?
>>>
>>> There isn't an architected way to probe whether a memory location has a
>>> VA->PA mapping. The tags are addressed by PA but you can't reach them if
>>> you get a page fault on the VA. So we either document the kasan_report()
>>> preconditions or, as you suggest, update addr_has_metadata() for the
>>> HW_TAGS case. Something like:
>>>
>>> return is_vmalloc_addr(virt) || virt_addr_valid(virt));
>>>
>>
>> This seems not working on arm64 because according to virt_addr_valid 0 is a
>> valid virtual address, in fact:
>>
>> __is_lm_address(0) == true && pfn_valid(virt_to_pfn(0)) == true.
>
> Ah, so __is_lm_address(0) is true. Maybe we should improve this since
> virt_to_pfn(0) doesn't make much sense.
>

How do you propose to improve it?

>> An option could be to make an exception for virtual address 0 in
>> addr_has_metadata() something like:
>>
>> static inline bool addr_has_metadata(const void *addr)
>> {
>> if ((u64)addr == 0)
>> return false;
>>
>> return (is_vmalloc_addr(addr) || virt_addr_valid(addr));
>> }
>
> As Andrey replied, passing a non-zero small value would still be
> incorrectly detected as valid.
>

I would like to remove the check completely and have virt_addr_valid(addr) to
return the right thing if possible.

I admit, yesterday evening I did not thing it through completely before posting
this code that had the sole purpose to open the discussion. I agree in principle
on what Andrey said as well (addr < PAGE_SIZE).

--
Regards,
Vincenzo

2021-01-20 18:00:37

[permalink] [raw]

Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Wed, Jan 20, 2021 at 04:16:02PM +0000, Vincenzo Frascino wrote:
> On 1/20/21 4:04 PM, Catalin Marinas wrote:
> > On Tue, Jan 19, 2021 at 08:35:49PM +0000, Vincenzo Frascino wrote:
> >> On 1/19/21 6:52 PM, Catalin Marinas wrote:
> >>> On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
> >>>> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
> >>>> <[email protected]> wrote:
> >>>>>
> >>>>> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
> >>>>> the address passed as a parameter.
> >>>>>
> >>>>> Add a comment to make sure that the preconditions to the function are
> >>>>> explicitly clarified.
> >>>>>
> >>>>> Note: An invalid address (e.g. NULL pointer address) passed to the
> >>>>> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
> >>>>>
> >>>>> Cc: Andrey Ryabinin <[email protected]>
> >>>>> Cc: Alexander Potapenko <[email protected]>
> >>>>> Cc: Dmitry Vyukov <[email protected]>
> >>>>> Cc: Leon Romanovsky <[email protected]>
> >>>>> Cc: Andrey Konovalov <[email protected]>
> >>>>> Signed-off-by: Vincenzo Frascino <[email protected]>
> >>>>> ---
> >>>>> mm/kasan/report.c | 11 +++++++++++
> >>>>> 1 file changed, 11 insertions(+)
> >>>>>
> >>>>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> >>>>> index c0fb21797550..2485b585004d 100644
> >>>>> --- a/mm/kasan/report.c
> >>>>> +++ b/mm/kasan/report.c
> >>>>> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
> >>>>> end_report(&flags);
> >>>>> }
> >>>>>
> >>>>> +/**
> >>>>> + * kasan_report - report kasan fault details
> >>>>> + * @addr: valid address of the allocation where the tag fault was detected
> >>>>> + * @size: size of the allocation where the tag fault was detected
> >>>>> + * @is_write: the instruction that caused the fault was a read or write?
> >>>>> + * @ip: pointer to the instruction that cause the fault
> >>>>> + *
> >>>>> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
> >>>>> + * the address to access the tags, hence it must be valid at this point in
> >>>>> + * order to not cause a kernel panic.
> >>>>> + */
> >>>>
> >>>> It doesn't dereference the address, it just checks the tags, right?
> >>>>
> >>>> Ideally, kasan_report() should survive that with HW_TAGS like with the
> >>>> other modes. The reason it doesn't is probably because of a blank
> >>>> addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
> >>>> guess we should somehow check that the memory comes from page_alloc or
> >>>> kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
> >>>> instruction to check whether the memory has tags?
> >>>
> >>> There isn't an architected way to probe whether a memory location has a
> >>> VA->PA mapping. The tags are addressed by PA but you can't reach them if
> >>> you get a page fault on the VA. So we either document the kasan_report()
> >>> preconditions or, as you suggest, update addr_has_metadata() for the
> >>> HW_TAGS case. Something like:
> >>>
> >>> return is_vmalloc_addr(virt) || virt_addr_valid(virt));
> >>>
> >>
> >> This seems not working on arm64 because according to virt_addr_valid 0 is a
> >> valid virtual address, in fact:
> >>
> >> __is_lm_address(0) == true && pfn_valid(virt_to_pfn(0)) == true.
> >
> > Ah, so __is_lm_address(0) is true. Maybe we should improve this since
> > virt_to_pfn(0) doesn't make much sense.
>
> How do you propose to improve it?

Check that it's actually a kernel address starting at PAGE_OFFSET. The
current __is_lm_address() check just masks out the top 12 bits but if
they were 0, this still yields a true result. Maybe extending the
current definition as:

#define __is_lm_address(addr) ((u64)(addr) >= PAGE_OFFSET && \
((u64)(addr) & ~PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))

Which basically means:

#define __is_lm_address(addr) ((u64)(addr) >= PAGE_OFFSET && \
(u64)(addr) < PAGE_END)

I think we could write the above as:

#define __is_lm_address(addr) (((u64)(addr) ^ PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))

This way we catch any 0 bits in the top 12 (or 16 with a 48-bit VA
configuration).

--
Catalin

2021-01-21 11:35:45

[permalink] [raw]

Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

Hi Andrey,

On 1/19/21 8:56 PM, Andrey Konovalov wrote:
>> return (is_vmalloc_addr(addr) || virt_addr_valid(addr));
> Do we need is_vmalloc_addr()? As we don't yet have vmalloc support for HW_TAGS.

It is not necessary but it does not hurt, since we are going to add vmalloc
anyway at some point, I would keep it here.

--
Regards,
Vincenzo

2021-01-21 12:56:42