netdev - Re: [PATCH] net: micro-optimize skb_datagram

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Date: Sun, 16 Jun 2024 22:56:30 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Sagi Grimberg <sagi@...mberg.me>
Cc: kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
	lkp@...el.com, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH] net: micro-optimize skb_datagram_iter

On Sun, Jun 16, 2024 at 12:24:28PM +0300, Sagi Grimberg wrote:
> > [   13.495377][  T189] ------------[ cut here ]------------
> > [   13.495862][  T189] kernel BUG at mm/usercopy.c:102!
> > [   13.496372][  T189] Oops: invalid opcode: 0000 [#1] PREEMPT SMP
> > [   13.496927][  T189] CPU: 0 PID: 189 Comm: systemctl Not tainted 6.10.0-rc2-00258-g18f0423b9ecc #1
> > [   13.497741][  T189] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> > [ 13.498663][ T189] EIP: usercopy_abort (mm/usercopy.c:102 (discriminator 12))
> > [   13.499424][  T194] usercopy: Kernel memory exposure attempt detected from kmap (offset 0, size 8192)!
> 
> Hmm, not sure I understand exactly why changing kmap() to kmap_local_page()
> expose this,
> but it looks like mm/usercopy does not like size=8192 when copying for the
> skb frag.
> 
> quick git browse directs to:
> --
> commit 4e140f59d285c1ca1e5c81b4c13e27366865bd09
> Author: Matthew Wilcox (Oracle) <willy@...radead.org>
> Date:   Mon Jan 10 23:15:27 2022 +0000
> 
>     mm/usercopy: Check kmap addresses properly
> 
>     If you are copying to an address in the kmap region, you may not copy
>     across a page boundary, no matter what the size of the underlying
>     allocation.  You can't kmap() a slab page because slab pages always
>     come from low memory.
> 
>     Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>     Acked-by: Kees Cook <keescook@...omium.org>
>     Signed-off-by: Kees Cook <keescook@...omium.org>
>     Link:
> https://lore.kernel.org/r/20220110231530.665970-2-willy@infradead.org
> --
> 
> CCing willy.
> 
> The documentation suggest that under single-context usage kmap() can be
> freely converted
> to kmap_local_page()? But seems that when using kmap() the size is not an
> issue, still trying to
> understand why.

Probably because kmap() returns page_address() for non-highmem pages
while kmap_local_page() actually returns a kmap address:

        if (!IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) && !PageHighMem(page))
                return page_address(page);
        return __kmap_local_pfn_prot(page_to_pfn(page), prot);

so if skb frags are always lowmem (are they?) this is a false positive.
if they can be highmem, then you've uncovered a bug that nobody's
noticed because nobody's testing on 32-bit any more.