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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