Re: [PATCH] off-by-one in get_mempolicy(2)

[Vlastimil Babka <vbabka@suse.cz>]

On 10/9/19 4:05 PM, Al Viro wrote:
> 	get_mempolicy(2) and related syscalls have always passed
> 1 + number of bits in nodemask as maxnodes argument - see e.g.
> copy_nodes_to_user() and get_nodes().  Or libnuma, for the userland
> side -
> static void getpol(int *oldpolicy, struct bitmask *bmp)
> {
>         if (get_mempolicy(oldpolicy, bmp->maskp, bmp->size + 1, 0, 0) < 0)
>                 numa_error("get_mempolicy");
> }
> and similar for other syscalls.  However, the check for insufficient
> destination size in get_mempolicy(2) used to be
>         if (nmask != NULL && maxnode < MAX_NUMNODES)
>                 return -EINVAL;
> IOW, maxnode == MAX_NUMNODES (representing "MAX_NUMNODES - 1 bits")
> had been accepted.  The reason why that hadn't messed libnuma
> logics used to determine the required bitmap size is that
> MAX_NUMNODES is always a power of 2 and the loop in libnuma
> is
>                 nodemask_sz = 16;
>                 do {
>                         nodemask_sz <<= 1;
>                         mask = realloc(mask, nodemask_sz / 8);
>                         if (!mask)
>                                 return;
>                 } while (get_mempolicy(&pol, mask, nodemask_sz + 1, 0, 0) < 0 && errno == EINVAL &&
>                                 nodemask_sz < 4096*8);
> I.e. it's been passing 33, 65, 127, etc. until it got it large enough.

Sigh, it was silly of me to hope nobody is doing that [1]. I thought
libnuma was parsing /proc/self/status though, IIRC I've checked [2]

> That sidesteps the boundary case - we never try to pass exactly
> MAX_NUMNODES there.
> 
> However, that has changed recently, when get_mempolicy() switched
> to 
>         if (nmask != NULL && maxnode < nr_node_ids)
>                 return -EINVAL;
> _That_ can trigger.  Consider a box with nr_node_ids == 65.
> The first call in libnuma:set_nodemask_size() loop will
> pass 33 and fail, then we'll raise nodemask_sz to 64,
> allocate a 64bit mask and call get_mempolicy(&pol, mask, 65, 0, 0),
> which will succeed.  OK, so we decide to use 64bit bitmaps, and
> subsequent getpol() will be passing 65 to get_mempolicy(2).  Which
> is not a good idea, since kernel-side we'll get
> 	copy_nodes_to_user(nmask, 65, &nodes)
> And that will copy only 8 bytes out of kernel-side bitmap with
> 65 bits in it...
> 
> IOW, that check always should had been <=, not <; it didn't matter
> until commit 050c17f239fd ("numa: change get_mempolicy() to use
> nr_node_ids instead of MAX_NUMNODES") this year.  The fix is trivial
> - we need to make that check consistent with the code that does
> actual copyin/copyout.
> 
> Fixes: 050c17f239fd ("numa: change get_mempolicy() to use nr_node_ids instead of MAX_NUMNODES")
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

We should have reverted 050c17f239fd as it was fixing a patch in mmotm
that was ultimately discarded. It's not ideal e.g. for CRIU to determine
maxnode on old system and keep the value even on a new system with
possibly more nodes. But the commit was too quickly pushed into stables,
complicating the situation.

If we're not reverting then
Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks.

[1]
https://lore.kernel.org/linux-mm/32575d26-b141-6985-833a-12d48c0dce6a@suse.cz/
[2]
https://lore.kernel.org/linux-mm/4dab8a83-803a-56e0-6bbf-bdf581f2d1b4@suse.cz/

> ---
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 4ae967bcf954..e184df7633b0 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1561,7 +1561,7 @@ static int kernel_get_mempolicy(int __user *policy,
>  
>  	addr = untagged_addr(addr);
>  
> -	if (nmask != NULL && maxnode < nr_node_ids)
> +	if (nmask != NULL && maxnode <= nr_node_ids)
>  		return -EINVAL;
>  
>  	err = do_get_mempolicy(&pval, &nodes, addr, flags);
>