[linux-morello] Re: [PATCH] [WIP]binfmt_elf: Restrict CHERI auxv permissions

On 02/05/2023 12:09, Teo Couprie Diaz wrote:

Retrict the permisions of the CHERI specific auxv members as defined in the PCuABI specification.

Signed-off-by: Teo Couprie Diaz teo.coupriediaz@arm.com

This is not complete by any mean and could be used as a starting point. This does work for restricting the permissions but I couldn't manage to restrict the bounds in a similar fashion.

FWIW it only really makes sense to do this once we've got reservations in place. At that point it will just be a matter of using cheri_build_user_cap(), splitting each reservation (executable/interpreter) in two (RX/RW).

Kevin

fs/binfmt_elf.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index 1d82465cb9e9..96b4e1794c6c 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c @@ -308,21 +308,29 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec, } #if defined(CONFIG_CHERI_PURECAP_UABI) && (ELF_COMPAT == 0) /*

• * TODO [PCuABI] - Restrict bounds/perms for AT_CHERI_* entries
  

• * TODO [PCuABI] - Restrict bounds for AT_CHERI_* entries
  

  */ NEW_AUX_ENT(AT_CHERI_EXEC_RW_CAP, (exec_load_info->start_elf_rw != ~0UL ?

• 	elf_uaddr_to_user_ptr(exec_load_info->start_elf_rw) :
  

• 	cheri_perms_and(elf_uaddr_to_user_ptr(exec_load_info->start_elf_rw),
  

• 		(CHERI_PERMS_ROOTCAP | CHERI_PERMS_READ | CHERI_PERMS_WRITE |
  

• 		 ARM_CAP_PERMISSION_BRANCH_SEALED_PAIR)) :
  NULL));
  

  NEW_AUX_ENT(AT_CHERI_EXEC_RX_CAP,

• elf_uaddr_to_user_ptr(exec_load_info->start_elf_rx));
  

• cheri_perms_and(elf_uaddr_to_user_ptr(exec_load_info->start_elf_rx),
  

• 	(CHERI_PERMS_ROOTCAP | CHERI_PERMS_READ | CHERI_PERMS_EXEC |
  

• 	 ARM_CAP_PERMISSION_BRANCH_SEALED_PAIR)));
  

  NEW_AUX_ENT(AT_CHERI_INTERP_RW_CAP, ((interp_load_addr && interp_load_info->start_elf_rw != ~0UL) ?

• 	elf_uaddr_to_user_ptr(interp_load_info->start_elf_rw) :
  

• 	cheri_perms_and(elf_uaddr_to_user_ptr(interp_load_info->start_elf_rw),
  

• 		(CHERI_PERMS_ROOTCAP | CHERI_PERMS_READ | CHERI_PERMS_WRITE |
  

• 		 ARM_CAP_PERMISSION_BRANCH_SEALED_PAIR)) :
  NULL));
  

  NEW_AUX_ENT(AT_CHERI_INTERP_RX_CAP, (interp_load_addr ?

• 	elf_uaddr_to_user_ptr(interp_load_info->start_elf_rx) :
  

• 	cheri_perms_and(elf_uaddr_to_user_ptr(interp_load_info->start_elf_rx),
  

• 		(CHERI_PERMS_ROOTCAP | CHERI_PERMS_READ | CHERI_PERMS_EXEC |
  

• 		 ARM_CAP_PERMISSION_BRANCH_SEALED_PAIR)) :
  NULL));
  

  NEW_AUX_ENT(AT_CHERI_STACK_CAP, elf_uaddr_to_user_ptr(0)); NEW_AUX_ENT(AT_CHERI_SEAL_CAP, cheri_user_root_seal_cap);