On 2023-07-28, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
On 2023-07-26, Alexey Gladkov <legion@xxxxxxxxxx> wrote:On Wed, Jul 26, 2023 at 02:36:25AM +1000, Aleksa Sarai wrote:On 2023-07-11, Alexey Gladkov <legion@xxxxxxxxxx> wrote:On the userspace side fchmodat(3) is implemented as a wrapper function which implements the POSIX-specified interface. This interface differs from the underlying kernel system call, which does not have a flags argument. Most implementations require procfs [1][2]. There doesn't appear to be a good userspace workaround for this issue but the implementation in the kernel is pretty straight-forward. The new fchmodat2() syscall allows to pass the AT_SYMLINK_NOFOLLOW flag, unlike existing fchmodat. [1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=17eca54051ee28ba1ec3f9aed170a62630959143;hb=a492b1e5ef7ab50c6fdd4e4e9879ea5569ab0a6c#l35 [2] https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c?id=718f363bc2067b6487900eddc9180c84e7739f80#n28 Co-developed-by: Palmer Dabbelt <palmer@xxxxxxxxxx> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxx> Signed-off-by: Alexey Gladkov <legion@xxxxxxxxxx> Acked-by: Arnd Bergmann <arnd@xxxxxxxx> --- fs/open.c | 18 ++++++++++++++---- include/linux/syscalls.h | 2 ++ 2 files changed, 16 insertions(+), 4 deletions(-) diff --git a/fs/open.c b/fs/open.c index 0c55c8e7f837..39a7939f0d00 100644 --- a/fs/open.c +++ b/fs/open.c @@ -671,11 +671,11 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode) return err; } -static int do_fchmodat(int dfd, const char __user *filename, umode_t mode) +static int do_fchmodat(int dfd, const char __user *filename, umode_t mode, int lookup_flags)I think it'd be much neater to do the conversion of AT_ flags here and pass 0 as a flags argument for all of the wrappers (this is how most of the other xyz(), fxyz(), fxyzat() syscall wrappers are done IIRC).I just addressed the Al Viro's suggestion. https://lore.kernel.org/lkml/20190717014802.GS17978@xxxxxxxxxxxxxxxxxx/I think Al misspoke, because he also said "pass it 0 as an extra argument", but you actually have to pass LOOKUP_FOLLOW from the wrappers. If you look at how faccessat2 and faccessat are implemented, it follows the behaviour I described.{ struct path path; int error; - unsigned int lookup_flags = LOOKUP_FOLLOW; + retry: error = user_path_at(dfd, filename, lookup_flags, &path); if (!error) { @@ -689,15 +689,25 @@ static int do_fchmodat(int dfd, const char __user *filename, umode_t mode) return error; } +SYSCALL_DEFINE4(fchmodat2, int, dfd, const char __user *, filename, + umode_t, mode, int, flags) +{ + if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW)) + return -EINVAL;We almost certainly want to support AT_EMPTY_PATH at the same time. Otherwise userspace will still need to go through /proc when trying to chmod a file handle they have.I'm not sure I understand. Can you explain what you mean?You should add support for AT_EMPTY_PATH (LOOKUP_EMPTY) as well as AT_SYMLINK_NOFOLLOW. It would only require something like: unsigned int lookup_flags = LOOKUP_FOLLOW; if (flags & ~(AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW)) return -EINVAL; if (flags & AT_EMPTY_PATH) lookup_flags |= LOOKUP_EMPTY; if (flags & AT_SYMLINK_NOFOLLOW) lookup_flags &= ~LOOKUP_FOLLOW; /* ... */ This would be effectively equivalent to fchmod(fd, mode). (I was wrong when I said this wasn't already possible -- I forgot about fchmod(2).)
... with the exception (as Christian mentioned) of O_PATH descriptors. However, there are two counter-points to this: * fchownat(AT_EMPTY_PATH) exists but fchown() doesn't work on O_PATH descriptors *by design* (according to open(2)). * chmod(/proc/self/fd/$n) works on O_PATH descriptors, meaning this behaviour is already allowed and all that AT_EMPTY_PATH would do is allow programs to avoid depending on procfs for this. FWIW, I agree with Christian that these behaviours are not ideal (and I'm working on a series that might allow for these things to be properly blocked in the future) but there's also the consistency argument -- I don't think fchownat() is much safer to allow in this way than fchmodat() and (again) this behaviour is already possible through procfs. Ultimately, we can always add AT_EMPTY_PATH later. It just seemed like an obvious omission to me that would be easy to resolve. -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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