Re: [PATCH] mm: Consider __GFP_NOWARN flag for oversized kvmalloc() calls

[Michal Hocko]

On Fri 04-03-22 15:26:32, Daniel Borkmann wrote:
> syzkaller was recently triggering an oversized kvmalloc() warning via
> xdp_umem_create().
> 
> The triggered warning was added back in 7661809d493b ("mm: don't allow
> oversized kvmalloc() calls"). The rationale for the warning for huge
> kvmalloc sizes was as a reaction to a security bug where the size was
> more than UINT_MAX but not everything was prepared to handle unsigned
> long sizes.
> 
> Anyway, the AF_XDP related call trace from this syzkaller report was:
> 
>   kvmalloc include/linux/mm.h:806 [inline]
>   kvmalloc_array include/linux/mm.h:824 [inline]
>   kvcalloc include/linux/mm.h:829 [inline]
>   xdp_umem_pin_pages net/xdp/xdp_umem.c:102 [inline]
>   xdp_umem_reg net/xdp/xdp_umem.c:219 [inline]
>   xdp_umem_create+0x6a5/0xf00 net/xdp/xdp_umem.c:252
>   xsk_setsockopt+0x604/0x790 net/xdp/xsk.c:1068
>   __sys_setsockopt+0x1fd/0x4e0 net/socket.c:2176
>   __do_sys_setsockopt net/socket.c:2187 [inline]
>   __se_sys_setsockopt net/socket.c:2184 [inline]
>   __x64_sys_setsockopt+0xb5/0x150 net/socket.c:2184
>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>   do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Björn mentioned that requests for >2GB allocation can still be valid:
> 
>   The structure that is being allocated is the page-pinning accounting.
>   AF_XDP has an internal limit of U32_MAX pages, which is *a lot*, but
>   still fewer than what memcg allows (PAGE_COUNTER_MAX is a LONG_MAX/
>   PAGE_SIZE on 64 bit systems). [...]
> 
>   I could just change from U32_MAX to INT_MAX, but as I stated earlier
>   that has a hacky feeling to it. [...] From my perspective, the code
>   isn't broken, with the memcg limits in consideration. [...]
> 
> Linus says:
> 
>   [...] Pretty much every time this has come up, the kernel warning has
>   shown that yes, the code was broken and there really wasn't a reason
>   for doing allocations that big.
> 
>   Of course, some people would be perfectly fine with the allocation
>   failing, they just don't want the warning. I didn't want __GFP_NOWARN
>   to shut it up originally because I wanted people to see all those
>   cases, but these days I think we can just say "yeah, people can shut
>   it up explicitly by saying 'go ahead and fail this allocation, don't
>   warn about it'".
> 
>   So enough time has passed that by now I'd certainly be ok with [it].
> 
> Thus allow call-sites to silence such userspace triggered splats if the
> allocation requests have __GFP_NOWARN. For xdp_umem_pin_pages()'s call
> to kvcalloc() this is already the case, so nothing else needed there.
> 
> Fixes: 7661809d493b ("mm: don't allow oversized kvmalloc() calls")
> Reported-by: syzbot+11421fbbff99b989670e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Tested-by: syzbot+11421fbbff99b989670e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: Björn Töpel <bjorn@xxxxxxxxxx>
> Cc: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
> Cc: Willy Tarreau <w@xxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/bpf/CAJ+HfNhyfsT5cS_U9EC213ducHs9k9zNxX9+abqC0kTrPbQ0gg@xxxxxxxxxxxxxx
> Link: https://lore.kernel.org/bpf/20211201202905.b9892171e3f5b9a60f9da251@xxxxxxxxxxxxxxxxxxxx

This makes sense to me.
Ackd-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  [ Hi Linus, just to follow-up on the discussion from here [0], I've cooked
>    up proper and tested patch. Feel free to take it directly to your tree if
>    you prefer, or we could also either route it via bpf or mm, whichever way
>    is best. Thanks!
>    [0] https://lore.kernel.org/bpf/CAHk-=wiRq+_jd_O1gz3J6-ANtXMY7iLpi8XFUcmtB3rBixvUXQ@xxxxxxxxxxxxxx/ ]
> 
>  mm/util.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 7e43369064c8..d3102081add0 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -587,8 +587,10 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
>  		return ret;
>  
>  	/* Don't even allow crazy sizes */
> -	if (WARN_ON_ONCE(size > INT_MAX))
> +	if (unlikely(size > INT_MAX)) {
> +		WARN_ON_ONCE(!(flags & __GFP_NOWARN));
>  		return NULL;
> +	}
>  
>  	return __vmalloc_node(size, 1, flags, node,
>  			__builtin_return_address(0));
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs