From: Christophe Leroy > Sent: 02 September 2020 14:25 > Le 02/09/2020 à 15:13, David Laight a écrit : > > From: Christoph Hellwig > >> Sent: 02 September 2020 13:37 > >> > >> On Wed, Sep 02, 2020 at 08:15:12AM +0200, Christophe Leroy wrote: > >>>> - return 0; > >>>> - return (size == 0 || size - 1 <= seg.seg - addr); > >>>> + if (addr >= TASK_SIZE_MAX) > >>>> + return false; > >>>> + if (size == 0) > >>>> + return false; > >>> > >>> __access_ok() was returning true when size == 0 up to now. Any reason to > >>> return false now ? > >> > >> No, this is accidental and broken. Can you re-run your benchmark with > >> this fixed? > > > > Is TASK_SIZE_MASK defined such that you can do: > > > > return (addr | size) < TASK_SIZE_MAX) || !size; > > TASK_SIZE_MAX will usually be 0xc0000000 > > With: > addr = 0x80000000; > size = 0x80000000; > > I expect it to fail .... > > With the formula you propose it will succeed, won't it ? Hmmm... Was i getting confused about some comments for 64bit about there being such a big hole between valid user and kernel addresses that it was enough to check that 'size < TASK_SIZE_MAX'. That would be true for 64bit x86 (and probably ppc (& arm??)) if TASK_SIZE_MAX were 0x4 << 60. IIUC the highest user address is (much) less than 0x0 << 60 and the lowest kernel address (much) greater than 0xf << 60 on all these 64bit platforms. Actually if doing access_ok() inside get_user() you don't need to check the size at all. You don't even need to in copy_to/from_user() provided it always does a forwards copy. (Rather that copying the last word first for misaligned lengths.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
