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To: Al Viro Cc: linux-fsdevel@vger.kernel.org, brauner@kernel.org, io-uring@vger.kernel.org, cgzones@googlemail.com References: <20241002011011.GB4017910@ZenIV> <20241002012230.4174585-1-viro@zeniv.linux.org.uk> <20241002012230.4174585-5-viro@zeniv.linux.org.uk> <12334e67-80a6-4509-9826-90d16483835e@kernel.dk> <20241002020857.GC4017910@ZenIV> <20241002211939.GE4017910@ZenIV> <20241006052859.GD4017910@ZenIV> Content-Language: en-US From: Jens Axboe In-Reply-To: <20241006052859.GD4017910@ZenIV> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/5/24 11:28 PM, Al Viro wrote: > On Wed, Oct 02, 2024 at 04:55:22PM -0600, Jens Axboe wrote: > >> The reason I liked the putname() is that it's unconditional - the caller >> can rely on it being put, regardless of the return value. So I'd say the >> same should be true for ctx.kvalue, and if not, the caller should still >> free it. That's the path of least surprise - no leak for the least >> tested error path, and no UAF in the success case. > > The problem with ctx.kvalue is that on the syscall side there's a case when > we do not call either file_setxattr() or filename_setxattr() - -EBADF. > And it's a lot more convenient to do setxattr_copy() first, so we end > up with a lovely landmine: > filename = getname_xattr(pathname, at_flags); > if (!filename) { > CLASS(fd, f)(dfd); > if (fd_empty(f)) { > kfree(ctx.kvalue); // lest we leak > return -EBADF; > } > return file_setxattr(fd_file(f), &ctx); > } > return filename_setxattr(dfd, filename, lookup_flags, &ctx); > > That's asking for trouble, obviously. So I think we ought to consume > filename (in filename_...()) case, leave struct file reference alone > (we have to - it might have been borrowed rather than cloned) and leave > ->kvalue unchanged. Yes, it ends up being more clumsy, but at least > it's consistent between the cases... > > As for consuming filename... On the syscall side it allows things like > SYSCALL_DEFINE3(mkdirat, int, dfd, const char __user *, pathname, umode_t, mode) > { > return do_mkdirat(dfd, getname(pathname), mode); > } > which is better than the alternatives - I mean, that's > SYSCALL_DEFINE3(mkdirat, int, dfd, const char __user *, pathname, umode_t, mode) > { > struct filename *filename = getname(pathname); > int res = do_mkdirat(dfd, filename, mode); > putname(filename); > return ret; > } > or > SYSCALL_DEFINE3(mkdirat, int, dfd, const char __user *, pathname, umode_t, mode) > { > struct filename *filename __free(putname) = getname(pathname); > return do_mkdirat(dfd, filename, mode); > } > and both stink, if for different reasons ;-/ Having those things consume > (unconditionally) is better, IMO. > > Hell knows; let's go with what I described above for now and see where > it leads when more such helpers are regularized. Sounds like a plan. >>> Questions on the io_uring side: >>> * you usually reject REQ_F_FIXED_FILE for ...at() at ->prep() time. >>> Fine, but... what's the point of doing that in IORING_OP_FGETXATTR case? >>> Or IORING_OP_GETXATTR, for that matter, since you pass AT_FDCWD anyway... >>> Am I missing something subtle here? >> >> Right, it could be allowed for fgetxattr on the io_uring side. Anything >> that passes in a struct file would be fair game to enable it on. >> Anything that passes in a path (eg a non-fd value), it obviously >> wouldn't make sense anyway. > > OK, done and force-pushed into #work.xattr. I just checked, and while I think this is fine to do for the 'fd' taking {s,g}etxattr, I don't think the path taking ones should allow IOSQE_FIXED_FILE being set. It's nonsensical, as they don't take a file descriptor. So I'd prefer if we kept it to just the f* variants. I can just make this tweak in my io_uring 6.12 branch and get it upstream this week, that'll take it out of your hands. What do you think? -- Jens Axboe