[PATCH 4/3] binfmt/elf: don't expose prandom_u32() state

Yann Droneaud
2019-06-13T15:12:57Z

Using prandom_u32() to get random offsets might expose fraction
of its internal state to userspace;

To prevent leaking prandom_u32() state, get_random_u32() could be
used instead, but with greater cost.

But it would be a big waste to call get_random_u32() to retrieve
only 4bits to 8bits at a time.

Instead this patch makes use of get_random_u64() to seed once a
local PRNG.

The local PRNG can be used safely to produces the random offsets,
exposing its internal state won't harm.

Link: https://lore.kernel.org/lkml/cover.1560423331.git.ydroneaud@opteya.com
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
---
fs/binfmt_elf.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index c84ef81f0639..9aaca1f671d1 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -182,7 +182,8 @@ static inline elf_addr_t __user *elf_stack_alloc(unsigned long *pp,
return sp;
}

-static inline void elf_stack_randomize(unsigned long *pp, size_t range)
+static inline void elf_stack_randomize(unsigned long *pp,
+ struct rnd_state *state, size_t range)
{
u32 offset;
unsigned long p;
@@ -190,7 +191,7 @@ static inline void elf_stack_randomize(unsigned long *pp, size_t range)
if (!(current->flags & PF_RANDOMIZE))
return;

- offset = prandom_u32() % range;
+ offset = prandom_u32_state(state) % range;
p = *pp;

#ifdef CONFIG_STACK_GROWSUP
@@ -202,6 +203,15 @@ static inline void elf_stack_randomize(unsigned long *pp, size_t range)
*pp = p;
}

+static inline void elf_stack_randomize_seed(struct rnd_state *state)
+{
+ if (!(current->flags & PF_RANDOMIZE))
+ return;
+
+ prandom_seed_state(state,
+ get_random_u64());
+}
+
#ifndef ELF_BASE_PLATFORM
/*
* AT_BASE_PLATFORM indicates the "real" hardware/microarchitecture.
@@ -230,6 +240,9 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
int ei_index = 0;
const struct cred *cred = current_cred();
struct vm_area_struct *vma;
+ struct rnd_state state;
+
+ elf_stack_randomize_seed(&state);

/*
* In some cases (e.g. Hyper-Threading), we want to avoid L1
@@ -239,7 +252,7 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,

p = arch_align_stack(p);

- elf_stack_randomize(&p, 256);
+ elf_stack_randomize(&p, &state, 256);
elf_stack_align(&p);

/*
@@ -260,7 +273,7 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
if (k_platform) {
size_t len = strlen(k_platform) + 1;

- elf_stack_randomize(&p, 16);
+ elf_stack_randomize(&p, &state, 16);

u_platform = elf_stack_alloc(&p, len);
if (__copy_to_user(u_platform, k_platform, len))
@@ -275,14 +288,14 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
if (k_base_platform) {
size_t len = strlen(k_base_platform) + 1;

- elf_stack_randomize(&p, 16);
+ elf_stack_randomize(&p, &state, 16);

u_base_platform = elf_stack_alloc(&p, len);
if (__copy_to_user(u_base_platform, k_base_platform, len))
return -EFAULT;
}

- elf_stack_randomize(&p, 256);
+ elf_stack_randomize(&p, &state, 256);
elf_stack_align(&p);