Re: [RFC PATCH] mm: add probe_user_read() and probe_user_address()

[Christophe LEROY]

Le 19/10/2018 Ã 17:42, Kees Cook a ÃcritÂ:
> On Fri, Oct 19, 2018 at 8:14 AM, Christophe Leroy
> <christophe.leroy@xxxxxx> wrote:
> > In the powerpc, there are several places implementing safe
> > access to user data. This is sometimes implemented using
> > probe_kerne_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() and probe_kernel_address(),
> > this patch adds probe_user_read() and probe_user_address().
> >
> > probe_user_read() does the same as probe_kernel_read() but
> > first checks that it is really a user address.
> >
> > probe_user_address() is a shortcut to probe_user_read()
> >
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> >   include/linux/uaccess.h | 10 ++++++++++
> >   mm/maccess.c            | 33 +++++++++++++++++++++++++++++++++
> >   2 files changed, 43 insertions(+)
> >
> > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> > index efe79c1cdd47..fb00e3f847d7 100644
> > --- a/include/linux/uaccess.h
> > +++ b/include/linux/uaccess.h
> > @@ -266,6 +266,16 @@ 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_address(): safely attempt to read from a user location
> > + * @addr: address to read from
> > + * @retval: read into this variable
> > + *
> > + * Returns 0 on success, or -EFAULT.
> > + */
> > +#define probe_user_address(addr, retval)               \
> > +       probe_user_read(&(retval), addr, sizeof(retval))
> > +
> >   #ifndef user_access_begin
> >   #define user_access_begin() do { } while (0)
> >   #define user_access_end() do { } while (0)
> > diff --git a/mm/maccess.c b/mm/maccess.c
> > index ec00be51a24f..85d4a88a6917 100644
> > --- a/mm/maccess.c
> > +++ b/mm/maccess.c
> > @@ -67,6 +67,39 @@ long __probe_kernel_write(void *dst, const void *src, size_t size)
> >   EXPORT_SYMBOL_GPL(probe_kernel_write);
> >
> >   /**
> > + * 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.
> > + */
> > +
> > +long __weak probe_user_read(void *dst, const void *src, size_t size)
> > +       __attribute__((alias("__probe_user_read")));
>
> Let's use #defines to deal with per-arch aliases so we can keep the
> inline I'm suggesting below...
>
> > +
> > +long __probe_user_read(void *dst, const void __user *src, size_t size)
>
> Please make this __always_inline so the "size" variable can be
> examined for const-ness by the check_object_size() in
> __copy_from_user_inatomic().

Ok, I did as suggested in the patch I just sent.

Would it be worth doing the same with the existing probe_kernel_read() 
and probe_kernel_write() ?

Christophe

> -Kees