From mboxrd@z Thu Jan 1 00:00:00 1970 From: Aleksa Sarai Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Date: Thu, 5 Sep 2019 23:35:52 +1000 Message-ID: <20190905133552.xlckmxfzar4wh5ju@yavin.dot.cyphar.com> References: <20190904201933.10736-1-cyphar@cyphar.com> <20190904201933.10736-2-cyphar@cyphar.com> <20190905073205.GY2332@hirez.programming.kicks-ass.net> <20190905092622.tlb6nn3uisssdfbu@yavin.dot.cyphar.com> <20190905094305.GJ2349@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="p7vw24iv2smepbtp" Return-path: Content-Disposition: inline In-Reply-To: <20190905094305.GJ2349@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: Al Viro , Jeff Layton , "J. Bruce Fields" , Arnd Bergmann , David Howells , Shuah Khan , Shuah Khan , Ingo Molnar , Christian Brauner , Rasmus Villemoes , Eric Biederman , Andy Lutomirski , Andrew Morton , Alexei Starovoitov , Kees Cook , Jann Horn , Tycho Andersen , David Drysdale , Chanho Min , Oleg Nesterov , Alexander List-Id: linux-api@vger.kernel.org --p7vw24iv2smepbtp Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-09-05, Peter Zijlstra wrote: > On Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote: > > On 2019-09-05, Peter Zijlstra wrote: > > > On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote: > > > > +/** > > > > + * copy_struct_to_user: copy a struct to user space > > > > + * @dst: Destination address, in user space. > > > > + * @usize: Size of @dst struct. > > > > + * @src: Source address, in kernel space. > > > > + * @ksize: Size of @src struct. > > > > + * > > > > + * Copies a struct from kernel space to user space, in a way that = guarantees > > > > + * backwards-compatibility for struct syscall arguments (as long a= s future > > > > + * struct extensions are made such that all new fields are *append= ed* to the > > > > + * old struct, and zeroed-out new fields have the same meaning as = the old > > > > + * struct). > > > > + * > > > > + * @ksize is just sizeof(*dst), and @usize should've been passed b= y user space. > > > > + * The recommended usage is something like the following: > > > > + * > > > > + * SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, us= ize) > > > > + * { > > > > + * int err; > > > > + * struct foo karg =3D {}; > > > > + * > > > > + * // do something with karg > > > > + * > > > > + * err =3D copy_struct_to_user(uarg, usize, &karg, sizeof(kar= g)); > > > > + * if (err) > > > > + * return err; > > > > + * > > > > + * // ... > > > > + * } > > > > + * > > > > + * There are three cases to consider: > > > > + * * If @usize =3D=3D @ksize, then it's copied verbatim. > > > > + * * If @usize < @ksize, then kernel space is "returning" a newer= struct to an > > > > + * older user space. In order to avoid user space getting incom= plete > > > > + * information (new fields might be important), all trailing by= tes in @src > > > > + * (@ksize - @usize) must be zerored > > >=20 > > > s/zerored/zero/, right? > >=20 > > It should've been "zeroed". >=20 > That reads wrong to me; that way it reads like this function must take > that action and zero out the 'rest'; which is just wrong. >=20 > This function must verify those bytes are zero, not make them zero. Right, in my head I was thinking "must have been zeroed" which isn't what it says. I'll switch to "zero". > > > > , otherwise -EFBIG is retu= rned. > > >=20 > > > 'Funny' that, copy_struct_from_user() below seems to use E2BIG. > >=20 > > This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG = for > > a "too big" struct passed to the kernel, and EFBIG for a "too big" > > struct passed to user-space. I would personally have preferred EMSGSIZE > > instead of EFBIG, but felt using the existing error codes would be less > > confusing. >=20 > Sadly a recent commit: >=20 > 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify s= ched_read_attr() ABI logic and code") >=20 > Made the situation even 'worse'. I hadn't seen this patch before, and I have a few questions taking a look at it: * An error code for a particular behaviour was changed (EFBIG -> E2BIG). Is this not a userspace breakage (I know Linus went ballistic about something similar a while ago[1]), or did I misunderstand what the issue was in [1]? * At the risk of bike-shedding -- of we are changing it, wouldn't -EMSGSIZE be more appropriate? To be fair, picking errno values has always been more of an art than a science, but to my ears "Argument list too long" doesn't make too much sense in the context of "returning" a struct back to userspace (and the cause of the error is that the argument passed by user space *isn't big enough*). If there was an E2SMALL that would also work. ;) * Do you want me to write a patch based on that, to switch it to copy_struct_to_user()? * That patch removes the "are there non-zero bytes in the tail that userspace won't know about" check (which I have included in mine). I understand that this caused issues specifically with sched_getattr(2) due to the default value not being zero -- how should we rectify that (given that we'd hopefully want to port everyone who uses that interface to copy_struct_{to,from}_user())? * Given that the [uk]attr->size construct is pretty important to the usability of the sched and perf interfaces, should we require (or encourage) it for all struct-extension syscall setups? > > > > + if (unlikely(!access_ok(src, usize))) > > > > + return -EFAULT; > > > > + > > > > + /* Deal with trailing bytes. */ > > > > + if (usize < ksize) > > > > + memset(dst + size, 0, rest); > > > > + else if (usize > ksize) { > > > > + const void __user *addr =3D src + size; > > > > + char buffer[BUFFER_SIZE] =3D {}; > > >=20 > > > Isn't that too big for on-stack? > >=20 > > Is a 64-byte buffer too big? I picked the number "at random" to be the > > size of a cache line, but I could shrink it down to 32 bytes if the size > > is an issue (I wanted to avoid needless allocations -- hence it being > > on-stack). >=20 > Ah, my ctags gave me a definition of BUFFER_SIZE that was 512. I suppose > 64 should be OK. Good to know, though I'll rename it to avoid confusion. [1]: https://lore.kernel.org/lkml/CA+55aFy98A+LJK4+GWMcbzaa1zsPBRo76q+ioEjb= x-uaMKH6Uw@mail.gmail.com/ --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --p7vw24iv2smepbtp Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXXEPNQAKCRCdlLljIbnQ EtdDAQC347lG5qRdA84KUpGgbwgprjAxcKgxqQIULhRNFfpXbgD/ZCHtkcVeJovi WTsQxqcwA375UPIXJ/SgrKfqOJOI7Q4= =dSqz -----END PGP SIGNATURE----- --p7vw24iv2smepbtp--