Re: [PATCH 1/2] kmsan: simplify kmsan_internal_memmove_metadata()

[Alexander Potapenko]

On Mon, Sep 11, 2023 at 1:44 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Thu, 7 Sept 2023 at 15:06, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
> >
> > kmsan_internal_memmove_metadata() is the function that implements
> > copying metadata every time memcpy()/memmove() is called.
> > Because shadow memory stores 1 byte per each byte of kernel memory,
> > copying the shadow is trivial and can be done by a single memmove()
> > call.
> > Origins, on the other hand, are stored as 4-byte values corresponding
> > to every aligned 4 bytes of kernel memory. Therefore, if either the
> > source or the destination of kmsan_internal_memmove_metadata() is
> > unaligned, the number of origin slots corresponding to the source or
> > destination may differ:
> >
> >   1) memcpy(0xffff888080a00000, 0xffff888080900000, 4)
> >      copies 1 origin slot into 1 origin slot:
> >
> >      src (0xffff888080900000): xxxx
> >      src origins:              o111
> >      dst (0xffff888080a00000): xxxx
> >      dst origins:              o111
> >
> >   2) memcpy(0xffff888080a00001, 0xffff888080900000, 4)
> >      copies 1 origin slot into 2 origin slots:
> >
> >      src (0xffff888080900000): xxxx
> >      src origins:              o111
> >      dst (0xffff888080a00000): .xxx x...
> >      dst origins:              o111 o111
> >
> >   3) memcpy(0xffff888080a00000, 0xffff888080900001, 4)
> >      copies 2 origin slots into 1 origin slot:
> >
> >      src (0xffff888080900000): .xxx x...
> >      src origins:              o111 o222
> >      dst (0xffff888080a00000): xxxx
> >      dst origins:              o111
> >                            (or o222)
> >
> > Previously, kmsan_internal_memmove_metadata() tried to solve this
> > problem by copying min(src_slots, dst_slots) as is and cloning the
> > missing slot on one of the ends, if needed.
> > This was error-prone even in the simple cases where 4 bytes were copied,
> > and did not account for situations where the total number of nonzero
> > origin slots could have increased by more than one after copying:
> >
> >   memcpy(0xffff888080a00000, 0xffff888080900002, 8)
> >
> >   src (0xffff888080900002): ..xx .... xx..
> >   src origins:              o111 0000 o222
> >   dst (0xffff888080a00000): xx.. ..xx
> >                             o111 0000
> >                         (or 0000 o222)
> >
> > The new implementation simply copies the shadow byte by byte, and
> > updates the corresponding origin slot, if the shadow byte is nonzero.
> > This approach can handle complex cases with mixed initialized and
> > uninitialized bytes. Similarly to KMSAN inline instrumentation, latter
> > writes to bytes sharing the same origin slots take precedence.
> >
> > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
>
> I think this needs a Fixes tag.
Thanks, will add in v2!

> Also, is this corner case exercised by one of the KMSAN KUnit test cases?
Ditto

> Otherwise,
>
> Acked-by: Marco Elver <elver@xxxxxxxxxx>