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Sun, 23 Apr 2023 15:56:49 -0700 (PDT) Date: Sun, 23 Apr 2023 23:56:48 +0100 From: Lorenzo Stoakes To: Dave Chinner Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default Message-ID: <14c6f0f3-0747-4800-8718-4f109f7321ea@lucifer.local> References: <20230423222941.GR447837@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230423222941.GR447837@dread.disaster.area> X-Stat-Signature: m3mb7fsh44pfeasti4bcrgro7umc7kik X-Rspam-User: X-Rspamd-Queue-Id: 4C25440009 X-Rspamd-Server: rspam06 X-HE-Tag: 1682290612-2959 X-HE-Meta: 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 k8aeTiKp 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Apr 24, 2023 at 08:29:41AM +1000, Dave Chinner wrote: > On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote: > > +/* > > + * Writing to file-backed mappings using GUP is a fundamentally broken operation > > + * as kernel write access to GUP mappings may not adhere to the semantics > > + * expected by a file system. > > + * > > + * In most instances we disallow this broken behaviour, however there are some > > + * exceptions to this enforced here. > > + */ > > +static inline bool can_write_file_mapping(struct vm_area_struct *vma, > > + unsigned long gup_flags) > > +{ > > + struct file *file = vma->vm_file; > > + > > + /* If we aren't pinning then no problematic write can occur. */ > > + if (!(gup_flags & (FOLL_GET | FOLL_PIN))) > > + return true; > > + > > + /* Special mappings should pose no problem. */ > > + if (!file) > > + return true; > > Ok... > > > + > > + /* Has the caller explicitly indicated this case is acceptable? */ > > + if (gup_flags & FOLL_ALLOW_BROKEN_FILE_MAPPING) > > + return true; > > + > > + /* shmem and hugetlb mappings do not have problematic semantics. */ > > + return vma_is_shmem(vma) || is_file_hugepages(file); > > +} > > This looks backwards. We only want the override to occur when the > target won't otherwise allow it. i.e. This should be: > > if (vma_is_shmem(vma)) > return true; > if (is_file_hugepages(vma) > return true; > > /* > * Issue a warning only if we are allowing a write to a mapping > * that does not support what we are attempting to do functionality. > */ > if (WARN_ON_ONCE(gup_flags & FOLL_ALLOW_BROKEN_FILE_MAPPING)) > return true; > return false; > > i.e. we only want the warning to fire when the override is > triggered - indicating that the caller is actually using a file > mapping in a broken way, not when it is being used on > file/filesystem that actually supports file mappings in this way. > > > static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) > > { > > vm_flags_t vm_flags = vma->vm_flags; > > int write = (gup_flags & FOLL_WRITE); > > int foreign = (gup_flags & FOLL_REMOTE); > > + bool vma_anon = vma_is_anonymous(vma); > > > > if (vm_flags & (VM_IO | VM_PFNMAP)) > > return -EFAULT; > > > > - if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma)) > > + if ((gup_flags & FOLL_ANON) && !vma_anon) > > return -EFAULT; > > > > if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma)) > > @@ -978,6 +1008,10 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) > > return -EFAULT; > > > > if (write) { > > + if (!vma_anon && > > + WARN_ON_ONCE(!can_write_file_mapping(vma, gup_flags))) > > + return -EFAULT; > > Yeah, the warning definitely belongs in the check function when the > override triggers allow broken behaviour to proceed, not when we > disallow a write fault because the underlying file/filesystem does > not support the operation being attempted. I disagree for two reasons:- 1. There are places in the kernel that rely on this broken behaviour, most notably ptrace (and /proc/$pid/mem), but also the other places where you can see I've added this flag. I'm not sure spamming warnings for ordinary cases would be useful. 2. The purpose of putting a warning here is to catch any case I might have missed where broken behaviour is required, but now disallowed, because it might actually be hard for a GUP user to track down that this is why the GUP is no longer functioning (since all they'll see is an -EFAULT). This warned upon check should in reality not occur, because it implies the GUP user is trying to do something broken and is _not_ explicitly telling GUP that it knows it's doing it and can live with the consequences. And on that basis, is worthy of a warning so we know we have to go put this flag in that place (and know it is a source of problematic GUP usage), or fix the caller. An example case is placing breakpoints in gdb, without the flag being set for /proc/$pid/mem this will just fail. Raising a kernel warning when a user places a breakpoint seems... unhelpful :) > > -Dave. > -- > Dave Chinner > david@fromorbit.com