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Subject[PATCH bpf-next] bpf: Make trampolines W^X
Date
From: KP Singh <kpsingh@google.com>

The image for the BPF trampolines is allocated with
bpf_jit_alloc_exe_page which marks this allocated page executable. This
means that the allocated memory is W and X at the same time making it
susceptible to WX based attacks.

Since the allocated memory is shared between two trampolines (the
current and the next), 2 pages must be allocated to adhere to W^X and
the following sequence is obeyed where trampolines are modified:

- Mark memory as non executable (set_memory_nx). While module_alloc for
x86 allocates the memory as PAGE_KERNEL and not PAGE_KERNEL_EXEC, not
all implementations of module_alloc do so
- Mark the memory as read/write (set_memory_rw)
- Modify the trampoline
- Mark the memory as read-only (set_memory_ro)
- Mark the memory as executable (set_memory_x)

There's a window between the modification of the trampoline and setting
the trampoline as read-only where an attacker and ~could~ change the
contents of the memory. This can be further improved by adding more
verification after the memory is marked as read-only.

Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: KP Singh <kpsingh@google.com>
---
arch/x86/net/bpf_jit_comp.c | 2 +-
include/linux/bpf.h | 1 -
kernel/bpf/dispatcher.c | 30 +++++++++++++++++++++++----
kernel/bpf/trampoline.c | 41 +++++++++++++++++++------------------
4 files changed, 48 insertions(+), 26 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 4c8a2d1f8470..a510f8260322 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1527,7 +1527,7 @@ int arch_prepare_bpf_trampoline(void *image, struct btf_func_model *m, u32 flags
* Another half is an area for next trampoline.
* Make sure the trampoline generation logic doesn't overflow.
*/
- if (WARN_ON_ONCE(prog - (u8 *)image > PAGE_SIZE / 2 - BPF_INSN_SAFETY))
+ if (WARN_ON_ONCE(prog - (u8 *)image > PAGE_SIZE - BPF_INSN_SAFETY))
return -EFAULT;
return 0;
}
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index b14e51d56a82..3be8b1b0166d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -502,7 +502,6 @@ struct bpf_trampoline *bpf_trampoline_lookup(u64 key);
int bpf_trampoline_link_prog(struct bpf_prog *prog);
int bpf_trampoline_unlink_prog(struct bpf_prog *prog);
void bpf_trampoline_put(struct bpf_trampoline *tr);
-void *bpf_jit_alloc_exec_page(void);
#define BPF_DISPATCHER_INIT(name) { \
.mutex = __MUTEX_INITIALIZER(name.mutex), \
.func = &name##func, \
diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c
index 204ee61a3904..f4589da3bb34 100644
--- a/kernel/bpf/dispatcher.c
+++ b/kernel/bpf/dispatcher.c
@@ -93,13 +93,34 @@ int __weak arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs)
static int bpf_dispatcher_prepare(struct bpf_dispatcher *d, void *image)
{
s64 ips[BPF_DISPATCHER_MAX] = {}, *ipsp = &ips[0];
- int i;
+ int i, ret;

for (i = 0; i < BPF_DISPATCHER_MAX; i++) {
if (d->progs[i].prog)
*ipsp++ = (s64)(uintptr_t)d->progs[i].prog->bpf_func;
}
- return arch_prepare_bpf_dispatcher(image, &ips[0], d->num_progs);
+
+ /* First make the page non-executable and then make it RW to avoid it
+ * from being W+X. While x86's implementation of module_alloc
+ * allocates memory as non-executable, not all implementations do so.
+ * Till these are fixed, explicitly mark the memory as NX.
+ */
+ set_memory_nx((unsigned long)image, 1);
+ set_memory_rw((unsigned long)image, 1);
+
+ ret = arch_prepare_bpf_dispatcher(image, &ips[0], d->num_progs);
+ if (ret)
+ return ret;
+
+ /* First make the page read-only, and only then make it executable to
+ * prevent it from being W+X in between.
+ */
+ set_memory_ro((unsigned long)image, 1);
+ /* More checks can be done here to ensure that nothing was changed
+ * between arch_prepare_bpf_dispatcher and set_memory_ro.
+ */
+ set_memory_x((unsigned long)image, 1);
+ return 0;
}

static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
@@ -113,7 +134,7 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
noff = 0;
} else {
old = d->image + d->image_off;
- noff = d->image_off ^ (PAGE_SIZE / 2);
+ noff = d->image_off ^ PAGE_SIZE;
}

new = d->num_progs ? d->image + noff : NULL;
@@ -140,10 +161,11 @@ void bpf_dispatcher_change_prog(struct bpf_dispatcher *d, struct bpf_prog *from,

mutex_lock(&d->mutex);
if (!d->image) {
- d->image = bpf_jit_alloc_exec_page();
+ d->image = bpf_jit_alloc_exec(2 * PAGE_SIZE);
if (!d->image)
goto out;
}
+ set_vm_flush_reset_perms(d->image);

prev_num_progs = d->num_progs;
changed |= bpf_dispatcher_remove_prog(d, from);
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 505f4e4b31d2..ff6a92ef8945 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -14,22 +14,6 @@ static struct hlist_head trampoline_table[TRAMPOLINE_TABLE_SIZE];
/* serializes access to trampoline_table */
static DEFINE_MUTEX(trampoline_mutex);

-void *bpf_jit_alloc_exec_page(void)
-{
- void *image;
-
- image = bpf_jit_alloc_exec(PAGE_SIZE);
- if (!image)
- return NULL;
-
- set_vm_flush_reset_perms(image);
- /* Keep image as writeable. The alternative is to keep flipping ro/rw
- * everytime new program is attached or detached.
- */
- set_memory_x((long)image, 1);
- return image;
-}
-
struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
{
struct bpf_trampoline *tr;
@@ -50,12 +34,13 @@ struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
goto out;

/* is_root was checked earlier. No need for bpf_jit_charge_modmem() */
- image = bpf_jit_alloc_exec_page();
+ image = bpf_jit_alloc_exec(2 * PAGE_SIZE);
if (!image) {
kfree(tr);
tr = NULL;
goto out;
}
+ set_vm_flush_reset_perms(image);

tr->key = key;
INIT_HLIST_NODE(&tr->hlist);
@@ -125,14 +110,14 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
}

/* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
- * bytes on x86. Pick a number to fit into PAGE_SIZE / 2
+ * bytes on x86. Pick a number to fit into PAGE_SIZE.
*/
#define BPF_MAX_TRAMP_PROGS 40

static int bpf_trampoline_update(struct bpf_trampoline *tr)
{
- void *old_image = tr->image + ((tr->selector + 1) & 1) * PAGE_SIZE/2;
- void *new_image = tr->image + (tr->selector & 1) * PAGE_SIZE/2;
+ void *old_image = tr->image + ((tr->selector + 1) & 1) * PAGE_SIZE;
+ void *new_image = tr->image + (tr->selector & 1) * PAGE_SIZE;
struct bpf_prog *progs_to_run[BPF_MAX_TRAMP_PROGS];
int fentry_cnt = tr->progs_cnt[BPF_TRAMP_FENTRY];
int fexit_cnt = tr->progs_cnt[BPF_TRAMP_FEXIT];
@@ -160,6 +145,13 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
if (fexit_cnt)
flags = BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME;

+
+ /* First make the page non-executable and then make it RW to avoid it
+ * from being being W+X.
+ */
+ set_memory_nx((unsigned long)new_image, 1);
+ set_memory_rw((unsigned long)new_image, 1);
+
err = arch_prepare_bpf_trampoline(new_image, &tr->func.model, flags,
fentry, fentry_cnt,
fexit, fexit_cnt,
@@ -167,6 +159,15 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
if (err)
goto out;

+ /* First make the page read-only, and only then make it executable to
+ * prevent it from being W+X in between.
+ */
+ set_memory_ro((unsigned long)new_image, 1);
+ /* More checks can be done here to ensure that nothing was changed
+ * between arch_prepare_bpf_trampoline and set_memory_ro.
+ */
+ set_memory_x((unsigned long)new_image, 1);
+
if (tr->selector)
/* progs already running at this address */
err = modify_fentry(tr, old_image, new_image);
--
2.20.1
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