Hi, Davidlohr and Thomas, On Tue, Aug 17, 2021 at 3:03 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > On Mon, Aug 16 2021 at 11:27, Davidlohr Bueso wrote: > > On Mon, 16 Aug 2021, Huacai Chen wrote: > > > >>fault_in_user_writeable() should verify R/W access but only verify W. In > >>most archs W implies R, but not true in MIPS and LoongArch, so fix it. > > > > Yuck for a find_vma() in futex.c. If this is a problem in MIPS, shouldn't > > the fix be there? Furthermore it's stated that fault_in_user_writeable(): > > > > "Fault in user address and verify RW access" > > That seems to be wishful thinking given the fact that some architectures > do not imply R for FLAG_FAULT_WRITE. > > > And you guys seem to have proposed it already: > > > > https://lore.kernel.org/linux-mips/20200630005845.1239974-1-liulichao@xxxxxxxxxxx/ This works, but I don't think this is a MIPS problem, so does Thomas Bogendoerfer. Because write-only page is valid in MIPS (so Thomas rejected this patch). > > That's surely one way to fix that. If that does not work for whatever > reason, then we really don't want this find_vma() hack there, but rather > something like: I don't know why find_vma() is unacceptable here, there is also find_vma() in fixup_user_fault(). > > if (IS_ENABLED(CONFIG_ARCH_USER_FAULT_VOODOO) && get_user(&tmp, uaddr)) > return -EFAULT; get_user() may be better than find_vma(), but can we drop CONFIG_ARCH_USER_FAULT_VOODOO here? On those "W implies R" archs, get_user() always success, this can simplify the logic. Huacai > > Thanks, > > tglx
