On Thu, Feb 7, 2019 at 10:22 AM Christophe Leroy <christophe.le...@c-s.fr> wrote: > In powerpc code, there are several places implementing safe > access to user data. This is sometimes implemented using > probe_kernel_address() with additional access_ok() verification, > sometimes with get_user() enclosed in a pagefault_disable()/enable() > pair, etc. : > show_user_instructions() > bad_stack_expansion() > p9_hmi_special_emu() > fsl_pci_mcheck_exception() > read_user_stack_64() > read_user_stack_32() on PPC64 > read_user_stack_32() on PPC32 > power_pmu_bhrb_to() > > In the same spirit as probe_kernel_read(), this patch adds > probe_user_read(). > > probe_user_read() does the same as probe_kernel_read() but > first checks that it is really a user address. > > The patch defines this function as a static inline so the "size" > variable can be examined for const-ness by the check_object_size() > in __copy_from_user_inatomic() > > Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr>
> --- > v3: Moved 'Returns:" comment after description. > Explained in the commit log why the function is defined static inline > > v2: Added "Returns:" comment and removed probe_user_address() > > include/linux/uaccess.h | 34 ++++++++++++++++++++++++++++++++++ > 1 file changed, 34 insertions(+) > > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h > index 37b226e8df13..ef99edd63da3 100644 > --- a/include/linux/uaccess.h > +++ b/include/linux/uaccess.h > @@ -263,6 +263,40 @@ extern long strncpy_from_unsafe(char *dst, const void > *unsafe_addr, long count); > #define probe_kernel_address(addr, retval) \ > probe_kernel_read(&retval, addr, sizeof(retval)) > > +/** > + * probe_user_read(): safely attempt to read from a user location > + * @dst: pointer to the buffer that shall take the data > + * @src: address to read from > + * @size: size of the data chunk > + * > + * Safely read from address @src to the buffer at @dst. If a kernel fault > + * happens, handle that and return -EFAULT. > + * > + * We ensure that the copy_from_user is executed in atomic context so that > + * do_page_fault() doesn't attempt to take mmap_sem. This makes > + * probe_user_read() suitable for use within regions where the caller > + * already holds mmap_sem, or other locks which nest inside mmap_sem. > + * > + * Returns: 0 on success, -EFAULT on error. > + */ > + > +#ifndef probe_user_read > +static __always_inline long probe_user_read(void *dst, const void __user > *src, > + size_t size) > +{ > + long ret; > + > + if (!access_ok(src, size)) > + return -EFAULT; If this happens in code that's running with KERNEL_DS, the access_ok() is a no-op. If this helper is only intended for accessing real userspace memory, it would be more robust to add set_fs(USER_DS)/set_fs(oldfs) around this thing. Looking at the functions you're referring to in the commit message, e.g. show_user_instructions() does an explicit `__access_ok(pc, NR_INSN_TO_PRINT * sizeof(int), USER_DS)` to get the same effect. (However, __access_ok() looks like it's horribly broken on x86 from what I can tell, because it's going to use the generic version that always returns 1...) > + pagefault_disable(); > + ret = __copy_from_user_inatomic(dst, src, size); > + pagefault_enable(); > + > + return ret ? -EFAULT : 0; > +} > +#endif > + > #ifndef user_access_begin > #define user_access_begin(ptr,len) access_ok(ptr, len) > #define user_access_end() do { } while (0) > -- > 2.13.3 > >