Messages in this thread | | | Date | Thu, 7 Jan 2016 17:23:29 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it |
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On Thu, Jan 07, 2016 at 03:13:37PM +0100, Arnd Bergmann wrote: > On Wednesday 06 January 2016 17:10:47 Catalin Marinas wrote: > > On Tue, Jan 05, 2016 at 10:12:20PM +0100, Arnd Bergmann wrote: > > > On Tuesday 05 January 2016 18:26:57 Yury Norov wrote: > > > > > So the calling conventions avoid the problem of being able to set > > > > > the upper bits from malicious user space when the kernel assumes they > > > > > are zeroed out (we had security bugs in this area, before we introduced > > > > > SYSCALL_DEFINEx()), but it means that we need wrappers around each > > > > > syscall that takes an argument that is different length between user > > > > > and kernel space (as Catalin guessed). arch/s390 has the same problem and > > > > > works around it with code in arch/s390/kernel/compat_wrapper.c, while > > > > > other architectures (at least powerpc, x86 and tile IIRC, don't know much > > > > > about mips, parisc and sparc) don't have the problem because of their > > > > > calling conventions. > > > > > > > > > > This also means that we cannot work around it in glibc at all, because > > > > > we have to be able to handle malicious user space, so it has to be > > > > > done in the kernel using something similar to what s390 does. > > > > > > > > So it seems like we (should) have 2 compat modes - with and without access > > > > to upper half of register. I'm thinking now on how put it in generic > > > > unistd.h less painfull way. > > > > > > I think we can do that by slightly modifying the existing __SYSCALL/__SC_3264/ > > > __SC_COMP/__SC_COMP_3264 macros: The first two need extra wrappers for > > > arm64-ilp32 and s390, the other two don't. > > > > > > We can use some clever string concatenation to add a ##_wrapper to the name > > > of the handler where needed and then just have a file that implements > > > the wrappers, copied from s390. > > > > > > Unfortunately, we can't just zero out all the upper halves and be done with > > > it: even if we went back to passing 64-bit arguments as separate 32-bit > > > registers, we'd still need to deal with sign-extending negative 32-bit > > > numbers. > > > > How many syscalls would we need sign-extension for? Most are probably > > already handled by specific compat_sys_* functions, otherwise A32 compat > > wouldn't work properly. > > Good point. I suppose any system call that expects a negative argument > may run into this on all architectures and require a COMPAT_SYSCALL handler, > but only s390 cares about doing the extension for the entire set of syscalls. > > This may be to work around a peculiarity of s390, which has now two > but three possible 32-to-64 extension modes: signed int, unsigned int > and pointer. The third one sets the top 33 bits to zero, clearing the > top bit of the 31-bit pointer value in the process. Nothing else needs > this, so if we just clear the upper bits on all system calls and go > back to passing 64-bit arguments as pairs, we are fine and have a much > simpler solution.
It would be indeed simpler from a kernel perspective. I'm not sure about the performance impact (a wrapper which does "mov wn, wn" for the first 6 registers, on top of existing wrappers). OTOH, with explicit wrappers we have the overhead of an additional function call, so we may be better off with the former.
> > For native syscalls like sys_read, apart from pointers we also need to > > handle size_t. The wrapper would need to be defined using compat types: > > > > ILP32_SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, compat_size_t, count) > > > > and let the compiler handle the conversion to size_t automatically when > > calling sys_read from the wrapper. > > Correct. I don't think we need an ILP32_SYSCALL_DEFINEx set of macros > though, the existing COMPAT_SYSCALL_DEFINEx ones should get this right > already.
The existing COMPAT_SYSCALL_DEFINEx macros generate the wrapper and definition for the compat_sys_* functions. What I meant by an ILP32_SYSCALL_DEFINEx is a macro which only generates a wrapper that calls into the native syscall (after sanitizing the arguments).
-- Catalin
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