On 09/22/2016 07:07 PM, Eric Dumazet wrote:
On Thu, 2016-09-22 at 18:56 +0200, Vlastimil Babka wrote:
On 09/22/2016 06:49 PM, Eric Dumazet wrote:
> On Thu, 2016-09-22 at 18:43 +0200, Vlastimil Babka wrote:
>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
>> with the number of fds passed. We had a customer report page allocation
>> failures of order-4 for this allocation. This is a costly order, so it might
>> easily fail, as the VM expects such allocation to have a lower-order fallback.
>>
>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
>> physically contiguous. Also the allocation is temporary for the duration of the
>> syscall, so it's unlikely to stress vmalloc too much.
>
> vmalloc() uses a vmap_area_lock spinlock, and TLB flushes.
>
> So I guess allowing vmalloc() being called from an innocent application
> doing a select() might be dangerous, especially if this select() happens
> thousands of time per second.
Isn't seq_buf_alloc() similarly exposed? And ipc_alloc()?
Possibly.
We don't have a library function (attempting kmalloc(), fallback to
vmalloc() presumably to avoid abuses, but I guess some patches were
accepted without thinking about this.
So in the case of select() it seems like the memory we need 6 bits per file
descriptor, multiplied by the highest possible file descriptor (nfds) as passed
to the syscall. According to the man page of select:
EINVAL nfds is negative or exceeds the RLIMIT_NOFILE resource limit (see
getrlimit(2)).
The code actually seems to silently cap the value instead of returning EINVAL
though? (IIUC):
/* max_fds can increase, so grab it once to avoid race */
rcu_read_lock();
fdt = files_fdtable(current->files);
max_fds = fdt->max_fds;
rcu_read_unlock();
if (n > max_fds)
n = max_fds;
The default for this cap seems to be 1024 where I checked (again, IIUC, it's
what ulimit -n returns?). I wasn't able to change it to more than 2048, which
makes the bitmaps still below PAGE_SIZE.
So if I get that right, the system admin would have to allow really large
RLIMIT_NOFILE to even make vmalloc() possible here. So I don't see it as a large
concern?
Vlastimil
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