Date:   Wed, 14 Apr 2021 16:30:11 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Catalin Marinas <catalin.marinas@....com>
CC:     Andre Przywara <andre.przywara@....com>,
        Will Deacon <will@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>
Subject: Re: [PATCH] arch/arm64/kernel/traps: Use find_vma_intersection() in
 traps for setting si_code

* Catalin Marinas <catalin.marinas@....com> [210413 14:00]:
> On Tue, Apr 13, 2021 at 04:52:34PM +0000, Liam Howlett wrote:
> > * Catalin Marinas <catalin.marinas@....com> [210412 13:44]:
> > > On Wed, Apr 07, 2021 at 03:11:06PM +0000, Liam Howlett wrote:
> > > > find_vma() will continue to search upwards until the end of the virtual
> > > > memory space.  This means the si_code would almost never be set to
> > > > SEGV_MAPERR even when the address falls outside of any VMA.  The result
> > > > is that the si_code is not reliable as it may or may not be set to the
> > > > correct result, depending on where the address falls in the address
> > > > space.
> > > > 
> > > > Using find_vma_intersection() allows for what is intended by only
> > > > returning a VMA if it falls within the range provided, in this case a
> > > > window of 1.
> > > > 
> > > > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > > > ---
> > > >  arch/arm64/kernel/traps.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > > > index a05d34f0e82a..a44007904a64 100644
> > > > --- a/arch/arm64/kernel/traps.c
> > > > +++ b/arch/arm64/kernel/traps.c
> > > > @@ -383,9 +383,10 @@ void force_signal_inject(int signal, int code, unsigned long address, unsigned i
> > > >  void arm64_notify_segfault(unsigned long addr)
> > > >  {
> > > >  	int code;
> > > > +	unsigned long ut_addr = untagged_addr(addr);
> > > >  
> > > >  	mmap_read_lock(current->mm);
> > > > -	if (find_vma(current->mm, untagged_addr(addr)) == NULL)
> > > > +	if (find_vma_intersection(current->mm, ut_addr, ut_addr + 1) == NULL)
> > > >  		code = SEGV_MAPERR;
> > > >  	else
> > > >  		code = SEGV_ACCERR;
> [...]
> > > I don't think your change is entirely correct either. We can have a
> > > fault below the vma of a stack (with VM_GROWSDOWN) and
> > > find_vma_intersection() would return NULL but it should be a SEGV_ACCERR
> > > instead.
> > 
> > I'm pretty sure I am missing something.  From what you said above, I
> > think this means that there can be a user cache fault below the stack
> > which should notify the user application that they are not allowed to
> > expand the stack by sending a SIGV_ACCERR in the si_code?  Is this
> > expected behaviour or am I missing a code path to this function?
> 
> My point was that find_vma() may return a valid vma where addr < vm_end
> but also addr < vm_addr. It's the responsibility of the caller to check
> that that vma can be expanded (VM_GROWSDOWN) and we do something like
> this in __do_page_fault(). find_vma_intersection(), OTOH, requires addr
> >= vm_start.

Right.  The find_vma() interface is not clear by the function name;
returning a VMA that doesn't include the address of interest is unclear.
I think this is why we ended up with the bug in the first place.

> 
> If we hit this case (addr < vm_start), normally we'd first need to check
> whether it's expandable and, if not, return MAPERR. If it's expandable,
> it should be ACCERR since something else caused the fault.
> 
> Now, I think at least for user_cache_maint_handler(), we can assume that
> __do_page_fault() handled any expansion already, so we don't need to
> check it here. In this case, your find_vma_intersection() check should
> work.
> 
> Are there other cases where we invoke arm64_notify_segfault() without a
> prior fault? I think in swp_handler() we can bail out early before we
> even attempted the access so we may report MAPERR but ACCERR is a better
> indication.

swp_handler() is also buggy.  It is currently getting the ACCERR as long
as the address being checked is > mm->highest_vm_end.  If access_ok()
fails, it should return ACCERR and not search VMAs for the address at
all.

...

>Also in sys_rt_sigreturn() we always call it as
> arm64_notify_segfault(regs->sp). I'm not sure that's correct in all
> cases, see restore_altstack().

Ditto for sys_rt_sigreturn() and sys_sigreturn(), they both suffer the
same bug as swp_handler() outlined above.

In the case of restore_sigframe() or restore_altstack() failing, it
seems that the signal shouldn't be dependent on where the address falls
within the VMA at all.  Should the signal still be SIGSEGV or something
else?  By the comments, I would have thought SIGBUS, si_code of
BUS_ADRALN?

> 
> I guess this code needs some tidying up.

Indeed, there seems to be a few code paths that need to skip this
function all together and just set the code to ACCERR - especially since
the access_ok() just validates the number itself.

I don't think the right answer is to rewrite the function to check
VM_GROWSDOWN, as I cannot see a way to reach this function when trying
to expand the stack which should report back ACCERR.  Do you agree?

I also see that my fix would expose other bugs which need to be
addressed at the same time.

> 
> > > Maybe this should employ similar checks as __do_page_fault() (with
> > > expand_stack() and VM_GROWSDOWN).
> > 
> > You mean the code needs to detect endianness and to check if this is an
> > attempt to expand the stack for both cases?
> 
> Nothing to do with endianness, just the relation between the address and
> the vma->vm_start and whether the vma can be expanded down.

Okay, thanks for clarifying.


Cheers,
Liam

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