bpf: Recover arena kernel faults with scratch page

BPF arena usage is becoming more prevalent, but kernel <-> BPF communication
over arena memory is awkward today. Data has to be staged through a trusted
kernel pointer with extra code and copying on the BPF side. While reads
through arena pointers can use a fault-safe helper, writes don't have a good
solution. The in-line alternative would need instruction emulation or asm
fixup labels.

Enable direct kernel-side reads and writes within GUARD_SZ / 2 of any
handed-in arena pointer, without bounds checking. A per-arena scratch page
is installed by the arch fault path into empty arena kernel PTEs - x86 from
page_fault_oops() for not-present faults, arm64 from __do_kernel_fault() for
translation faults, both after the existing exception-table and KFENCE
handling. The faulting instruction retries and the access is also reported
through the program's BPF stream, preserving error reporting.

bpf_prog_find_from_stack() resolves the current BPF program (and its arena)
from the kernel stack - no new bpf_run_ctx state is added. Recovery covers
the 4 GiB arena plus the upper half-guard (GUARD_SZ / 2). The lower
half-guard is excluded because well-behaved kfuncs only access forward from
arena pointers. The kfunc-author contract - access at most GUARD_SZ / 2 past
a handed-in pointer - is documented in Documentation/bpf/kfuncs.rst.

The install is lock-free via ptep_try_set(). On race-loss the winning
installer's PTE is already valid, so the access retry succeeds. The arena
clear path uses ptep_get_and_clear() so installer and clearer race through
atomic accessors. No flush_tlb_kernel_range() afterwards. Stale "not mapped"
entries just cause one extra re-fault, cheaper than a global IPI on every
install.

Scratch exists only to keep the kernel from oopsing on an in-line arena
access. Its presence at a PTE means the BPF program has already
malfunctioned, and the violation is reported through the program's BPF
stream. The only requirement for behavior on a scratched PTE is that the
kernel doesn't crash. In particular, any user-side access through such a PTE
may segfault. The shared scratch page is freed once during map destruction.

BPF instruction faults continue to use the existing JIT exception-table
path. This patch changes only the kernel-text fault path. No UAPI flag is
added. The new behavior is the default.

v2: Use ptep_get_and_clear() in apply_range_clear_cb(). (David)
v3: Stub bpf_arena_handle_page_fault() for !CONFIG_BPF_SYSCALL. (lkp)

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Cc: David Hildenbrand <david@kernel.org>
Link: https://lore.kernel.org/r/20260522172219.1423324-3-tj@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 20 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 01e203964892..bb4261a5df64 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -6,6 +6,7 @@
 
 #include <uapi/linux/bpf.h>
 #include <uapi/linux/filter.h>
+#include <linux/bpf_defs.h>
 
 #include <crypto/sha2.h>
 #include <linux/workqueue.h>
diff --git a/include/linux/bpf_defs.h b/include/linux/bpf_defs.h
new file mode 100644
index 000000000000..2185cd3966d4
--- /dev/null
+++ b/include/linux/bpf_defs.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Subset of bpf.h declarations, split out so files that need only these
+ * declarations can avoid bpf.h's full include cost.
+ */
+#ifndef _LINUX_BPF_DEFS_H
+#define _LINUX_BPF_DEFS_H
+
+#ifdef CONFIG_BPF_SYSCALL
+bool bpf_arena_handle_page_fault(unsigned long addr, bool is_write, unsigned long fault_ip);
+#else
+static inline bool bpf_arena_handle_page_fault(unsigned long addr, bool is_write,
+					       unsigned long fault_ip)
+{
+	return false;
+}
+#endif
+
+#endif /* _LINUX_BPF_DEFS_H */