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

[Yann Droneaud <ydroneaud@opteya.com>]

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);
 
 	/* Create the ELF interpreter info */
-- 
2.21.0