Re: [PATCH] arch/arm64/kernel/traps: Use find_vma_intersection() in traps for setting si_code

[Liam Howlett]

* Catalin Marinas <catalin.marinas@xxxxxxx> [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@xxxxxxxxxx>
> > ---
> >  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;


Thank you for taking the time to thoroughly review this patch.

> 
> 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?

> 
> 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?

Thanks,
Liam