Re: [PATCH v2 1/2] mm: vmalloc: implement vrealloc()

[Feng Tang]

On Tue, Jul 30, 2024 at 08:15:34PM +0800, Vlastimil Babka wrote:
> On 7/30/24 3:35 AM, Danilo Krummrich wrote:
[...]
> > 
> > Maybe I spoke a bit to soon with this last paragraph. I think continuously
> > gowing something with __GFP_ZERO is a legitimate use case. I just did a quick
> > grep for users of krealloc() with __GFP_ZERO and found 18 matches.
> > 
> > So, I think, at least for now, we should instead document that __GFP_ZERO is
> > only fully honored when the buffer is grown continuously (without intermediate
> > shrinking) and __GFP_ZERO is supplied in every iteration.
> > 
> > In case I miss something here, and not even this case is safe, it looks like
> > we have 18 broken users of krealloc().
> 
> +CC Feng Tang

Sorry for the late reply!

> 
> Let's say we kmalloc(56, __GFP_ZERO), we get an object from kmalloc-64
> cache. Since commit 946fa0dbf2d89 ("mm/slub: extend redzone check to
> extra allocated kmalloc space than requested") and preceding commits, if
> slub_debug is enabled (red zoning or user tracking), only the 56 bytes
> will be zeroed. The rest will be either unknown garbage, or redzone.

Yes.

> 
> Then we might e.g. krealloc(120) and get a kmalloc-128 object and 64
> bytes (result of ksize()) will be copied, including the garbage/redzone.
> I think it's fixable because when we do this in slub_debug, we also
> store the original size in the metadata, so we could read it back and
> adjust how many bytes are copied.

krealloc() --> __do_krealloc() --> ksize()
When ksize() is called, as we don't know what user will do with the
extra space ([57, 64] here), the orig_size check will be unset by
__ksize() calling skip_orig_size_check(). 

And if the newsize is bigger than the old 'ksize', the 'orig_size'
will be correctly set for the newly allocated kmalloc object.

For the 'unstable' branch of -mm tree, which has all latest patches
from Danilo, I run some basic test and it seems to be fine. 

> 
> Then we could guarantee that if __GFP_ZERO is used consistently on
> initial kmalloc() and on krealloc() and the user doesn't corrupt the
> extra space themselves (which is a bug anyway that the redzoning is
> supposed to catch) all will be fine.
> 
> There might be also KASAN side to this, I see poison_kmalloc_redzone()
> is also redzoning the area between requested size and cache's object_size?

AFAIK, KASAN has 3 modes: generic, SW-taged, HW-tagged, while the
latter 2 modes relied on arm64. For 'generic' mode, poison_kmalloc_redzone()
only redzone its own shadow memory, and not the kmalloc object data
space [orig_size + 1, ksize]. For the other 2 modes, I have no hardware
to test, but I guess they are also fine, otherwise there should be
already some bug report :), as normal kmalloc() may call it too. 

Thanks,
Feng