Re: [RFC PATCH 1/4] powerpc/code-patching: add patch_memory() for writing RO text

[Russell Currey <ruscur@russell.cc>]

On Thu, 2022-09-01 at 07:01 +0000, Christophe Leroy wrote:
> 
> 
> Le 01/09/2022 à 07:58, Benjamin Gray a écrit :
> > From: Russell Currey <ruscur@russell.cc>
> > 
> > powerpc allocates a text poke area of one page that is used by
> > patch_instruction() to modify read-only text when STRICT_KERNEL_RWX
> > is enabled.
> > 
> > patch_instruction() is only designed for instructions,
> > so writing data using the text poke area can only happen 4 bytes
> > at a time - each with a page map/unmap, pte flush and syncs.
> > 
> > This patch introduces patch_memory(), implementing the same
> > interface as memcpy(), similar to x86's text_poke() and s390's
> > s390_kernel_write().  patch_memory() only needs to map the text
> > poke area once, unless the write would cross a page boundary.
> > 
> > Signed-off-by: Russell Currey <ruscur@russell.cc>
> > Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
> > ---
> >   arch/powerpc/include/asm/code-patching.h |  1 +
> >   arch/powerpc/lib/code-patching.c         | 65
> > ++++++++++++++++++++++++
> >   2 files changed, 66 insertions(+)
> > 
> > diff --git a/arch/powerpc/include/asm/code-patching.h
> > b/arch/powerpc/include/asm/code-patching.h
> > index 1c6316ec4b74..3de90748bce7 100644
> > --- a/arch/powerpc/include/asm/code-patching.h
> > +++ b/arch/powerpc/include/asm/code-patching.h
> > @@ -76,6 +76,7 @@ int create_cond_branch(ppc_inst_t *instr, const
> > u32 *addr,
> >   int patch_branch(u32 *addr, unsigned long target, int flags);
> >   int patch_instruction(u32 *addr, ppc_inst_t instr);
> >   int raw_patch_instruction(u32 *addr, ppc_inst_t instr);
> > +void *patch_memory(void *dest, const void *src, size_t size);
> > 
> >   static inline unsigned long patch_site_addr(s32 *site)
> >   {
> > diff --git a/arch/powerpc/lib/code-patching.c
> > b/arch/powerpc/lib/code-patching.c
> > index 6edf0697a526..0cca39af44cb 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -14,6 +14,7 @@
> >   #include <asm/page.h>
> >   #include <asm/code-patching.h>
> >   #include <asm/inst.h>
> > +#include <asm/cacheflush.h>
> > 
> >   static int __patch_instruction(u32 *exec_addr, ppc_inst_t instr,
> > u32 *patch_addr)
> >   {
> > @@ -183,6 +184,65 @@ static int do_patch_instruction(u32 *addr,
> > ppc_inst_t instr)
> > 
> >          return err;
> >   }
> > +
> > +static int do_patch_memory(void *dest, const void *src, size_t
> > size)
> > +{
> > +       int err;
> > +       unsigned long text_poke_addr, patch_addr;
> > +
> > +       text_poke_addr = (unsigned
> > long)__this_cpu_read(text_poke_area)->addr;
> > +
> > +       err = map_patch_area(dest, text_poke_addr);
> 
> This is not in line with the optimisation done by 
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220815114840.1468656-1-mpe@ellerman.id.au/

This patch hasn't changed since last year, thanks for the pointer.

> 
> > +       if (err)
> > +               return err;
> > +
> > +       patch_addr = text_poke_addr + offset_in_page(dest);
> > +       copy_to_kernel_nofault((u8 *)patch_addr, src, size);
> 
> copy_to_kernel_nofault() has a performance cost.
> 
> > +
> > +       flush_icache_range(patch_addr, size);
> 
> Is that needed ? We are patching data, not text.

It's necessary if it gets used to patch text, which it might.  Maybe we
should add a variable and only flush if the caller thinks it's needed.

The comment below should be updated for that too.

> > +       unmap_patch_area(text_poke_addr);
> > +
> > +       return 0;
> > +}
> > +
> > +/**
> > + * patch_memory - write data using the text poke area
> > + *
> > + * @dest:      destination address
> > + * @src:       source address
> > + * @size:      size in bytes
> > + *
> > + * like memcpy(), but using the text poke area. No atomicity
> > guarantees.
> > + * Do not use for instructions, use patch_instruction() instead.
> > + * Handles crossing page boundaries, though you shouldn't need to.
> > + *
> > + * Return value:
> > + *     @dest
> > + **/
> > +void *patch_memory(void *dest, const void *src, size_t size)
> > +{
> > +       int err;
> > +       unsigned long flags;
> > +       size_t written, write_size;
> > +
> > +       // If the poke area isn't set up, it's early boot and we
> > can just memcpy.
> > +       if (!this_cpu_read(text_poke_area))
> > +               return memcpy(dest, src, size);
> > +
> > +       for (written = 0; written < size; written += write_size) {
> > +               // Write as much as possible without crossing a
> > page boundary.
> > +               write_size = min_t(size_t, size - written,
> > +                                  PAGE_SIZE - offset_in_page(dest
> > + written));
> > +
> > +               local_irq_save(flags);
> > +               err = do_patch_memory(dest + written, src +
> > written, write_size);
> > +               local_irq_restore(flags);
> > +               if (err)
> > +                       return ERR_PTR(err);
> > +       }
> > +
> > +       return dest;
> > +}
> >   #else /* !CONFIG_STRICT_KERNEL_RWX */
> > 
> >   static int do_patch_instruction(u32 *addr, ppc_inst_t instr)
> > @@ -190,6 +250,11 @@ static int do_patch_instruction(u32 *addr,
> > ppc_inst_t instr)
> >          return raw_patch_instruction(addr, instr);
> >   }
> > 
> > +void *patch_memory(void *dest, const void *src, size_t size)
> > +{
> > +       return memcpy(dest, src, size);
> 
> In do_patch_memory() you have flush_icache_range(patch_addr, size);
> 
> If that's really needed there, why don't we need it here as well ?

Good point.  I might make the arguments

	(void *dest, const void *src, size_t size, bool text)

and only do the icache flush if text is true.

> 
> > +}
> > +
> >   #endif /* CONFIG_STRICT_KERNEL_RWX */
> > 
> >   __ro_after_init DEFINE_STATIC_KEY_FALSE(init_mem_is_free);
> > --
> > 2.37.2