Re: [PATCH AUTOSEL 6.6 05/13] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Kees Cook]

Please drop this from -stable -- it's part of a larger refactoring that
shouldn't be backported without explicit effort/testing.

-Kees

On Mon, Nov 06, 2023 at 06:14:18PM -0500, Sasha Levin wrote:
> From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> 
> [ Upstream commit 585a018627b4d7ed37387211f667916840b5c5ea ]
> 
> Implement a helper elf_load() that wraps elf_map() and performs all
> of the necessary work to ensure that when "memsz > filesz" the bytes
> described by "memsz > filesz" are zeroed.
> 
> An outstanding issue is if the first segment has filesz 0, and has a
> randomized location. But that is the same as today.
> 
> In this change I replaced an open coded padzero() that did not clear
> all of the way to the end of the page, with padzero() that does.
> 
> I also stopped checking the return of padzero() as there is at least
> one known case where testing for failure is the wrong thing to do.
> It looks like binfmt_elf_fdpic may have the proper set of tests
> for when error handling can be safely completed.
> 
> I found a couple of commits in the old history
> https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git,
> that look very interesting in understanding this code.
> 
> commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail")
> commit c6e2227e4a3e ("[SPARC64]: Missing user access return value checks in fs/binfmt_elf.c and fs/compat.c")
> commit 5bf3be033f50 ("v2.4.10.1 -> v2.4.10.2")
> 
> Looking at commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail"):
> >  commit 39b56d902bf35241e7cba6cc30b828ed937175ad
> >  Author: Pavel Machek <pavel@xxxxxx>
> >  Date:   Wed Feb 9 22:40:30 2005 -0800
> >
> >     [PATCH] binfmt_elf: clearing bss may fail
> >
> >     So we discover that Borland's Kylix application builder emits weird elf
> >     files which describe a non-writeable bss segment.
> >
> >     So remove the clear_user() check at the place where we zero out the bss.  I
> >     don't _think_ there are any security implications here (plus we've never
> >     checked that clear_user() return value, so whoops if it is a problem).
> >
> >     Signed-off-by: Pavel Machek <pavel@xxxxxxx>
> >     Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
> >     Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx>
> 
> It seems pretty clear that binfmt_elf_fdpic with skipping clear_user() for
> non-writable segments and otherwise calling clear_user(), aka padzero(),
> and checking it's return code is the right thing to do.
> 
> I just skipped the error checking as that avoids breaking things.
> 
> And notably, it looks like Borland's Kylix died in 2005 so it might be
> safe to just consider read-only segments with memsz > filesz an error.
> 
> Reported-by: Sebastian Ott <sebott@xxxxxxxxxx>
> Reported-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
> Closes: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@xxxxxxxxxxxxxx
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/87sf71f123.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Tested-by: Pedro Falcato <pedro.falcato@xxxxxxxxx>
> Signed-off-by: Sebastian Ott <sebott@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/20230929032435.2391507-1-keescook@xxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
>  1 file changed, 48 insertions(+), 63 deletions(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 7b3d2d4914073..2a615f476e44e 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -110,25 +110,6 @@ static struct linux_binfmt elf_format = {
>  
>  #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
>  
> -static int set_brk(unsigned long start, unsigned long end, int prot)
> -{
> -	start = ELF_PAGEALIGN(start);
> -	end = ELF_PAGEALIGN(end);
> -	if (end > start) {
> -		/*
> -		 * Map the last of the bss segment.
> -		 * If the header is requesting these pages to be
> -		 * executable, honour that (ppc32 needs this).
> -		 */
> -		int error = vm_brk_flags(start, end - start,
> -				prot & PROT_EXEC ? VM_EXEC : 0);
> -		if (error)
> -			return error;
> -	}
> -	current->mm->start_brk = current->mm->brk = end;
> -	return 0;
> -}
> -
>  /* 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
> @@ -406,6 +387,51 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
>  	return(map_addr);
>  }
>  
> +static unsigned long elf_load(struct file *filep, unsigned long addr,
> +		const struct elf_phdr *eppnt, int prot, int type,
> +		unsigned long total_size)
> +{
> +	unsigned long zero_start, zero_end;
> +	unsigned long map_addr;
> +
> +	if (eppnt->p_filesz) {
> +		map_addr = elf_map(filep, addr, eppnt, prot, type, total_size);
> +		if (BAD_ADDR(map_addr))
> +			return map_addr;
> +		if (eppnt->p_memsz > eppnt->p_filesz) {
> +			zero_start = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
> +				eppnt->p_filesz;
> +			zero_end = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
> +				eppnt->p_memsz;
> +
> +			/* Zero the end of the last mapped page */
> +			padzero(zero_start);
> +		}
> +	} else {
> +		map_addr = zero_start = ELF_PAGESTART(addr);
> +		zero_end = zero_start + ELF_PAGEOFFSET(eppnt->p_vaddr) +
> +			eppnt->p_memsz;
> +	}
> +	if (eppnt->p_memsz > eppnt->p_filesz) {
> +		/*
> +		 * Map the last of the segment.
> +		 * If the header is requesting these pages to be
> +		 * executable, honour that (ppc32 needs this).
> +		 */
> +		int error;
> +
> +		zero_start = ELF_PAGEALIGN(zero_start);
> +		zero_end = ELF_PAGEALIGN(zero_end);
> +
> +		error = vm_brk_flags(zero_start, zero_end - zero_start,
> +				     prot & PROT_EXEC ? VM_EXEC : 0);
> +		if (error)
> +			map_addr = error;
> +	}
> +	return map_addr;
> +}
> +
> +
>  static unsigned long total_mapping_size(const struct elf_phdr *phdr, int nr)
>  {
>  	elf_addr_t min_addr = -1;
> @@ -829,7 +855,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
>  	struct elf_phdr *elf_property_phdata = NULL;
>  	unsigned long elf_bss, elf_brk;
> -	int bss_prot = 0;
>  	int retval, i;
>  	unsigned long elf_entry;
>  	unsigned long e_entry;
> @@ -1040,33 +1065,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		if (elf_ppnt->p_type != PT_LOAD)
>  			continue;
>  
> -		if (unlikely (elf_brk > elf_bss)) {
> -			unsigned long nbyte;
> -
> -			/* There was a PT_LOAD segment with p_memsz > p_filesz
> -			   before this one. Map anonymous pages, if needed,
> -			   and clear the area.  */
> -			retval = set_brk(elf_bss + load_bias,
> -					 elf_brk + load_bias,
> -					 bss_prot);
> -			if (retval)
> -				goto out_free_dentry;
> -			nbyte = ELF_PAGEOFFSET(elf_bss);
> -			if (nbyte) {
> -				nbyte = ELF_MIN_ALIGN - nbyte;
> -				if (nbyte > elf_brk - elf_bss)
> -					nbyte = elf_brk - elf_bss;
> -				if (clear_user((void __user *)elf_bss +
> -							load_bias, nbyte)) {
> -					/*
> -					 * This bss-zeroing can fail if the ELF
> -					 * file specifies odd protections. So
> -					 * we don't check the return value
> -					 */
> -				}
> -			}
> -		}
> -
>  		elf_prot = make_prot(elf_ppnt->p_flags, &arch_state,
>  				     !!interpreter, false);
>  
> @@ -1162,7 +1160,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  			}
>  		}
>  
> -		error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt,
> +		error = elf_load(bprm->file, load_bias + vaddr, elf_ppnt,
>  				elf_prot, elf_flags, total_size);
>  		if (BAD_ADDR(error)) {
>  			retval = IS_ERR_VALUE(error) ?
> @@ -1217,10 +1215,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		if (end_data < k)
>  			end_data = k;
>  		k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
> -		if (k > elf_brk) {
> -			bss_prot = elf_prot;
> +		if (k > elf_brk)
>  			elf_brk = k;
> -		}
>  	}
>  
>  	e_entry = elf_ex->e_entry + load_bias;
> @@ -1232,18 +1228,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	start_data += load_bias;
>  	end_data += load_bias;
>  
> -	/* Calling set_brk effectively mmaps the pages that we need
> -	 * for the bss and break sections.  We must do this before
> -	 * mapping in the interpreter, to make sure it doesn't wind
> -	 * up getting placed where the bss needs to go.
> -	 */
> -	retval = set_brk(elf_bss, elf_brk, bss_prot);
> -	if (retval)
> -		goto out_free_dentry;
> -	if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
> -		retval = -EFAULT; /* Nobody gets to see this, but.. */
> -		goto out_free_dentry;
> -	}
> +	current->mm->start_brk = current->mm->brk = ELF_PAGEALIGN(elf_brk);
>  
>  	if (interpreter) {
>  		elf_entry = load_elf_interp(interp_elf_ex,
> -- 
> 2.42.0
> 

-- 
Kees Cook