RE: Subject: [PATCH v2] slab: kmalloc_size_roundup() must not return 0 for non-zero size

[David Laight]

From: Vlastimil Babka
> Sent: 11 September 2023 16:54
> 
> On 9/8/23 10:26, David Laight wrote:
> > From: Kees Cook
> >> Sent: 07 September 2023 20:38
> >>
> >> On Thu, Sep 07, 2023 at 12:42:20PM +0000, David Laight wrote:
> >> > The typical use of kmalloc_size_roundup() is:
> >> > 	ptr = kmalloc(sz = kmalloc_size_roundup(size), ...);
> >> > 	if (!ptr) return -ENOMEM.
> >> > This means it is vitally important that the returned value isn't
> >> > less than the argument even if the argument is insane.
> >> > In particular if kmalloc_slab() fails or the value is above
> >> > (MAX_ULONG - PAGE_SIZE) zero is returned and kmalloc() will return
> >> > it's single zero-length buffer.
> >> >
> >> > Fix by returning the input size on error or if the size exceeds
> >> > a 'sanity' limit.
> >> > kmalloc() will then return NULL is the size really is too big.
> >> >
> >> >
> >> > Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
> >> > Fixes: 05a940656e1eb ("slab: Introduce kmalloc_size_roundup()")
> >> > ---
> >> > v2:
> >> >     - Use KMALLOC_MAX_SIZE for upper limit.
> >> >       (KMALLOC_MAX_SIZE + 1 may give better code on some archs!)
> >> >     - Invert test for overlarge for consistency.
> >> >     - Put a likely() on result of kmalloc_slab().
> >> >
> >> >  mm/slab_common.c | 26 +++++++++++++-------------
> >> >  1 file changed, 13 insertions(+), 13 deletions(-)
> >> >
> >> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> >> > index cd71f9581e67..0fb7c7e19bad 100644
> >> > --- a/mm/slab_common.c
> >> > +++ b/mm/slab_common.c
> >> > @@ -747,22 +747,22 @@ size_t kmalloc_size_roundup(size_t size)
> >> >  {
> >> >  	struct kmem_cache *c;
> >> >
> >> > -	/* Short-circuit the 0 size case. */
> >> > -	if (unlikely(size == 0))
> >> > -		return 0;
> >>
> >> If we want to allow 0, let's just leave this case as-is: the compiler
> >> will optimize it against the other tests.
> >
> > I doubt the compiler will optimise it away - especially with
> > the unlikely().
> 
> Yeah I also think compiler can't do much optimizations except for build-time
> constant 0 here.

Only relevant if the code were inlined - and it isn't.
(and is probably a bit big.)
I'm not sure you'd want to expose kmalloc_slab() to the wider kernel.

OTOH, it could have an inline version for constants > KMALLOC_CACHE_SIZE.
But they may not happen often enough to make any difference.

> 
> > OTOH the explicit checks for (size && size <= LIMIT) do
> > get optimised to ((size - 1) <= LIMIT - 1) so become
> > a single compare.
> >
> > Then returning 'size' at the bottom means that zero is returned
> > in the arg is zero - which is fine.
> >
> >>
> >> > -	/* Short-circuit saturated "too-large" case. */
> >> > -	if (unlikely(size == SIZE_MAX))
> >> > -		return SIZE_MAX;
> >> > +	if (size && size <= KMALLOC_MAX_CACHE_SIZE) {
> >> > +		/*
> >> > +		 * The flags don't matter since size_index is common to all.
> >> > +		 * Neither does the caller for just getting ->object_size.
> >> > +		 */
> >> > +		c = kmalloc_slab(size, GFP_KERNEL, 0);
> >> > +		return likely(c) ? c->object_size : size;
> >>
> >> I would like to have this fail "safe". c should never be NULL here, so
> >> let's return "KMALLOC_MAX_SIZE + 1" to force failures.
> >
> > Why even try to force failure here?
> > The whole function is just an optimisation so that the caller
> > can use the spare space.
> >
> > The only thing it mustn't do is return a smaller value.
> 
> If "c" is NULL it means either the kernel build must be broken e.g. by
> somebody breaking the KMALLOC_MAX_CACHE_SIZE value, and we could just ignore
> c being NULL and let it crash because of that.
> But I think it can also be NULL due to trying to call kmalloc_size_roundup()
> too early, when kmalloc_caches array is not yet populated. Note if we call
> kmalloc() itself too early, we get a NULL as a result, AFAICS. I can imagine
> two scenarios:
> 
> - kmalloc_size_roundup() is called with result immediately fed to kmalloc()
> that happens too early, in that case we best should not crash on c being
> NULL and make sure the kmalloc() returns NULL.
> - kmalloc_size_roundup() is called in some init code to get a value that
> some later kmalloc() call uses. We might want also not crash in that case,
> but informing the developer that they did something wrong would be also useful?
> 
> Clearly returning 0 if c == NULL, as done currently, is wrong for both
> scenarios. Retuning "size" is OK for the first scenario, also valid for the
> second one, but the caller will silently lose the benefit of
> kmalloc_size_roundup() and the developer introducing that won't realize it's
> done too early and could be fixed.

I'm sure that won't matter.

> So perhaps the best would be to return size for c == NULL, but also do a
> WARN_ONCE?

That would add a real function call to an otherwise leaf function
and almost certainly require the compiler create a stack frame.

...

I did have an interesting 'lateral thought' idea.
It is all very silly doing all the work twice, what you really
want is kmalloc() to return both the pointer and actual size.
But returning a 'two word' structure is done by reference and
would kill performance/
OTOH a lot of archs can return two word integers in a register pair
(dx:ax on x86).
Could you have the real function return ((unsigned __int64)size << 64 | (long)ptr)
and then extract the size in a wrapper macro?
(With different types for 32bit)

That will, of course, break the 'it's like malloc' checks the
compiler is doing - unless it is taught what is going on.

	David

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