On Fri, Feb 25, 2022 at 02:24:35PM -0500, Theodore Ts'o wrote: > [un]pin_user_pages_remote is dirtying pages without properly warning > the file system in advance (or faulting in the file data if the page > is not yet in the page cache). This was noted by Jan Kara in 2018[1] > and more recently has resulted in bug reports by Syzbot in various > Android kernels[2]. > > This is technically a bug in the mm/gup.c codepath, but arguably ext4 > is fragile in that a buggy get_user_pages() implementation causes ext4 > to crash, where as other file systems are not crashing (although in > some cases the user data will be lost since gup code is not properly > informing the file system to potentially allocate blocks or reserve > space when writing into a sparse portion of file). I suspect in real > life it is rare that people are using RDMA into file-backed memory, > which is why no one has complained to ext4 developers except fuzzing > programs. > > So instead of crashing with a BUG, issue a warning (since there may be > potential data loss) and just mark the page as clean to avoid > unprivileged denial of service attacks until the problem can be > properly fixed. More discussion and background can be found in the > thread starting at [2]. > > [1] https://www.spinics.net/lists/linux-mm/msg142700.html Can you use a lore link (https://lore.kernel.org/linux-mm/20180103100430.GE4911@xxxxxxxxxxxxxx/T/#u)? > + /* > + * Should never happen but for buggy code in > + * other subsystemsa that call subsystemsa => subsystems > + * set_page_dirty() without properly warning > + * the file system first. See [1] for more > + * information. > + * > + * [1] https://www.spinics.net/lists/linux-mm/msg142700.html Likewise, lore link here. - Eric
