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binfmt_elf: Support segments with 0 filesz and misaligned starts 

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@ucw.cz>
>  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@suse.cz>
>     Signed-off-by: Andrew Morton <akpm@osdl.org>
>     Signed-off-by: Linus Torvalds <torvalds@osdl.org>

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@redhat.com>
Reported-by: Thomas Weißschuh <linux@weissschuh.net>
Closes: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/r/87sf71f123.fsf@email.froward.int.ebiederm.org
Tested-by: Pedro Falcato <pedro.falcato@gmail.com>
Signed-off-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20230929032435.2391507-1-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
Eric W. Biederman 2023-09-28 20:24:29 -07:00 committed by Kees Cook
parent ff7a65492c
commit 585a018627
1 changed files with 48 additions and 63 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

111
fs/binfmt_elf.c
View File

@ -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 @@ out_free_interp:
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 @@ out_free_interp:
}
}
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 @@ out_free_interp:
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 @@ out_free_interp:
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,