Re: [PATCH v3 3/4] binfmt_elf: Provide prot bits as context for padzero() errors

[Kees Cook <keescook@chromium.org>]

On Wed, Sep 27, 2023 at 03:18:34PM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > Errors with padzero() should be caught unless we're expecting a
> > pathological (non-writable) segment. Report -EFAULT only when PROT_WRITE
> > is present.
> >
> > Additionally add some more documentation to padzero(), elf_map(), and
> > elf_load().
> 
> I wonder if this might be easier to just perform the PROT_WRITE
> test in elf_load, and to completely skip padzero of PROT_WRITE
> is not present. 

Yeah, actually, after moving load_elf_library() to elf_load(), there's
only 1 caller of padzero... :P

I'll work on that.

-Kees

> 
> Eric
> 
> > Cc: Eric Biederman <ebiederm@xmission.com>
> > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: linux-fsdevel@vger.kernel.org
> > Cc: linux-mm@kvack.org
> > Suggested-by: Eric Biederman <ebiederm@xmission.com>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  fs/binfmt_elf.c | 33 +++++++++++++++++++++++----------
> >  1 file changed, 23 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index 0214d5a949fc..b939cfe3215c 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -110,19 +110,21 @@ static struct linux_binfmt elf_format = {
> >  
> >  #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
> >  
> > -/* We need to explicitly zero any fractional pages
> > -   after the data section (i.e. bss).  This would
> > -   contain the junk from the file that should not
> > -   be in memory
> > +/*
> > + * We need to explicitly zero any trailing portion of the page that follows
> > + * p_filesz when it ends before the page ends (e.g. bss), otherwise this
> > + * memory will contain the junk from the file that should not be present.
> >   */
> > -static int padzero(unsigned long elf_bss)
> > +static int padzero(unsigned long address, int prot)
> >  {
> >  	unsigned long nbyte;
> >  
> > -	nbyte = ELF_PAGEOFFSET(elf_bss);
> > +	nbyte = ELF_PAGEOFFSET(address);
> >  	if (nbyte) {
> >  		nbyte = ELF_MIN_ALIGN - nbyte;
> > -		if (clear_user((void __user *) elf_bss, nbyte))
> > +		/* Only report errors when the segment is writable. */
> > +		if (clear_user((void __user *)address, nbyte) &&
> > +		    prot & PROT_WRITE)
> >  			return -EFAULT;
> >  	}
> >  	return 0;
> > @@ -348,6 +350,11 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
> >  	return 0;
> >  }
> >  
> > +/*
> > + * Map "eppnt->p_filesz" bytes from "filep" offset "eppnt->p_offset"
> > + * into memory at "addr". (Note that p_filesz is rounded up to the
> > + * next page, so any extra bytes from the file must be wiped.)
> > + */
> >  static unsigned long elf_map(struct file *filep, unsigned long addr,
> >  		const struct elf_phdr *eppnt, int prot, int type,
> >  		unsigned long total_size)
> > @@ -387,6 +394,11 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
> >  	return(map_addr);
> >  }
> >  
> > +/*
> > + * Map "eppnt->p_filesz" bytes from "filep" offset "eppnt->p_offset"
> > + * into memory at "addr". Memory from "p_filesz" through "p_memsz"
> > + * rounded up to the next page is zeroed.
> > + */
> >  static unsigned long elf_load(struct file *filep, unsigned long addr,
> >  		const struct elf_phdr *eppnt, int prot, int type,
> >  		unsigned long total_size)
> > @@ -405,7 +417,8 @@ static unsigned long elf_load(struct file *filep, unsigned long addr,
> >  				eppnt->p_memsz;
> >  
> >  			/* Zero the end of the last mapped page */
> > -			padzero(zero_start);
> > +			if (padzero(zero_start, prot))
> > +				return -EFAULT;
> >  		}
> >  	} else {
> >  		map_addr = zero_start = ELF_PAGESTART(addr);
> > @@ -712,7 +725,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
> >  	 * the file up to the page boundary, and zero it from elf_bss
> >  	 * up to the end of the page.
> >  	 */
> > -	if (padzero(elf_bss)) {
> > +	if (padzero(elf_bss, bss_prot)) {
> >  		error = -EFAULT;
> >  		goto out;
> >  	}
> > @@ -1407,7 +1420,7 @@ static int load_elf_library(struct file *file)
> >  		goto out_free_ph;
> >  
> >  	elf_bss = eppnt->p_vaddr + eppnt->p_filesz;
> > -	if (padzero(elf_bss)) {
> > +	if (padzero(elf_bss, PROT_WRITE)) {
> >  		error = -EFAULT;
> >  		goto out_free_ph;
> >  	}

-- 
Kees Cook