bpf: Support for hardening against JIT spraying

The BPF JIT allocator packs many small programs into larger executable
allocations and reuses space within those allocations as programs are
loaded and freed. When fresh code is written into space that a previous
program occupied, an indirect jump into the new program can reuse a branch
prediction left behind by the old one.

Flush the indirect branch predictors before reusing JIT memory so that
indirect jumps into a newly written program don't reuse predictions from an
old program that occupied the same space.

Introduce bpf_arch_pred_flush_enabled static key and bpf_arch_pred_flush
static call for flushing the branch predictors on JIT memory reuse.
Architectures that need a flush, can update it to a predictor flush
function. By default, its a NOP and does not emit any CALL.

Allocations larger than a pack are not covered by this flush. That is safe
because cBPF programs (the unprivileged attack surface) are bounded well
below a pack size. Issue a warning if this assumption is ever violated
while the flush is active.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 19 insertions, 0 deletions

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 649cce41e13f..7f0a17f128d4 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -20,6 +20,7 @@
 #include <uapi/linux/btf.h>
 #include <linux/filter.h>
 #include <linux/skbuff.h>
+#include <linux/static_call.h>
 #include <linux/vmalloc.h>
 #include <linux/prandom.h>
 #include <linux/bpf.h>
@@ -883,6 +884,15 @@ void bpf_jit_fill_hole_with_zero(void *area, unsigned int size)
 	memset(area, 0, size);
 }
 
+DEFINE_STATIC_CALL_NULL(bpf_arch_pred_flush, bpf_arch_pred_flush);
+
+/*
+ * Enabled once bpf_arch_pred_flush points at a real flush routine. Lets the
+ * pack allocator test "is a predictor flush wired up at all" with a cheap
+ * static branch instead of repeatedly querying the static call target.
+ */
+DEFINE_STATIC_KEY_FALSE(bpf_pred_flush_enabled);
+
 #define BPF_PROG_SIZE_TO_NBITS(size)	(round_up(size, BPF_PROG_CHUNK_SIZE) / BPF_PROG_CHUNK_SIZE)
 
 static DEFINE_MUTEX(pack_mutex);
@@ -941,6 +951,14 @@ void *bpf_prog_pack_alloc(u32 size, bpf_jit_fill_hole_t bpf_fill_ill_insns)
 
 	mutex_lock(&pack_mutex);
 	if (size > BPF_PROG_PACK_SIZE) {
+		/*
+		 * Allocations larger than a pack get their own pages, and
+		 * predictors are not flushed for such allocation. This is only
+		 * safe because cBPF programs (the unprivileged attack surface)
+		 * are bounded well below a pack size.
+		 */
+		if (static_branch_unlikely(&bpf_pred_flush_enabled))
+			pr_warn_once("BPF: Predictors not flushed for allocations greater than BPF_PROG_PACK_SIZE\n");
 		size = round_up(size, PAGE_SIZE);
 		ptr = bpf_jit_alloc_exec(size);
 		if (ptr) {
@@ -971,6 +989,7 @@ void *bpf_prog_pack_alloc(u32 size, bpf_jit_fill_hole_t bpf_fill_ill_insns)
 	pos = 0;
 
 found_free_area:
+	static_call_cond(bpf_arch_pred_flush)();
 	bitmap_set(pack->bitmap, pos, nbits);
 	ptr = (void *)(pack->ptr) + (pos << BPF_PROG_CHUNK_SHIFT);