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binfmt_elf: mseal address zero

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In load_elf_binary as part of the execve(), when the current
task’s personality has MMAP_PAGE_ZERO set, the kernel allocates
one page at address 0. According to the comment:

/* Why this, you ask???  Well SVr4 maps page 0 as read-only,
    and some applications "depend" upon this behavior.
    Since we do not have the power to recompile these, we
     emulate the SVr4 behavior. Sigh. */

At one point, Linus suggested removing this [1].

Code search in debian didn't see much use of MMAP_PAGE_ZERO [2],
it exists in util and test (rr).

Sealing this is probably safe, the comment doesn't say
the app ever wanting to change the mapping to rwx. Sealing
also ensures that never happens.

If there is a complaint, we can make this configurable.

Link: https://lore.kernel.org/lkml/CAHk-=whVa=nm_GW=NVfPHqcxDbWt4JjjK1YWb0cLjO4ZSGyiDA@mail.gmail.com/ [1]
Link: https://codesearch.debian.net/search?q=MMAP_PAGE_ZERO&literal=1&perpkg=1&page=1 [2]
Signed-off-by: Jeff Xu <jeffxu@chromium.org>
Link: https://lore.kernel.org/r/20240806214931.2198172-2-jeffxu@google.com
Signed-off-by: Kees Cook <kees@kernel.org>
This commit is contained in:
Jeff Xu 2024-08-06 21:49:27 +00:00 committed by Kees Cook
parent 7d442a33bf
commit 44f65d9006
3 changed files with 16 additions and 1 deletions
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5
fs/binfmt_elf.c
View File

@ -1314,6 +1314,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
emulate the SVr4 behavior. Sigh. */
error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
MAP_FIXED | MAP_PRIVATE, 0);
retval = do_mseal(0, PAGE_SIZE, 0);
if (retval)
pr_warn_ratelimited("pid=%d, couldn't seal address 0, ret=%d.\n",
task_pid_nr(current), retval);
}
regs = current_pt_regs();

10
include/linux/mm.h
View File

@ -4201,4 +4201,14 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma);
int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
#ifdef CONFIG_64BIT
int do_mseal(unsigned long start, size_t len_in, unsigned long flags);
#else
static inline int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
{
/* noop on 32 bit */
return 0;
}
#endif
#endif /* _LINUX_MM_H */

2
mm/mseal.c
View File

@ -248,7 +248,7 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
*
* unseal() is not supported.
*/
static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
{
size_t len;
int ret = 0;