Re: [PATCH 07/32] mm: Bring back vmalloc_exec

Kent Overstreet
2023-05-17T05:28:58Z

On Tue, May 16, 2023 at 10:47:13PM +0100, Matthew Wilcox wrote:
> On Tue, May 16, 2023 at 05:20:33PM -0400, Kent Overstreet wrote:
> > On Tue, May 16, 2023 at 02:02:11PM -0700, Kees Cook wrote:
> > > For something that small, why not use the text_poke API?
> >
> > This looks like it's meant for patching existing kernel text, which
> > isn't what I want - I'm generating new functions on the fly, one per
> > btree node.
> >
> > I'm working up a new allocator - a (very simple) slab allocator where
> > you pass a buffer, and it gives you a copy of that buffer mapped
> > executable, but not writeable.
> >
> > It looks like we'll be able to convert bpf, kprobes, and ftrace
> > trampolines to it; it'll consolidate a fair amount of code (particularly
> > in bpf), and they won't have to burn a full page per allocation anymore.
> >
> > bpf has a neat trick where it maps the same page in two different
> > locations, one is the executable location and the other is the writeable
> > location - I'm stealing that.
>
> How does that avoid the problem of being able to construct an arbitrary
> gadget that somebody else will then execute? IOW, what bpf has done
> seems like it's working around & undoing the security improvements.
>
> I suppose it's an improvement that only the executable address is
> passed back to the caller, and not the writable address.

Ok, here's what I came up with. Have not tested all corner cases, still
need to write docs - but I think this gives us a nicer interface than
what bpf/kprobes/etc. have been doing, and it does the sub-page sized
allocations I need.

With an additional tweak to module_alloc() (not done in this patch yet)
we avoid ever mapping in pages both writeable and executable:

-->--

From 6eeb6b8ef4271ea1a8d9cac7fbaeeb7704951976 Mon Sep 17 00:00:00 2001
From: Kent Overstreet <kent.overstreet@linux.dev>
Date: Wed, 17 May 2023 01:22:06 -0400
Subject: [PATCH] mm: jit/text allocator

This provides a new, very simple slab allocator for jit/text, i.e. bpf,
ftrace trampolines, or bcachefs unpack functions.

With this API we can avoid ever mapping pages both writeable and
executable (not implemented in this patch: need to tweak
module_alloc()), and it also supports sub-page sized allocations.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

diff --git a/include/linux/jitalloc.h b/include/linux/jitalloc.h
new file mode 100644
index 0000000000..f1549d60e8
--- /dev/null
+++ b/include/linux/jitalloc.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_JITALLOC_H
+#define _LINUX_JITALLOC_H
+
+void jit_update(void *buf, void *new_buf, size_t len);
+void jit_free(void *buf);
+void *jit_alloc(void *buf, size_t len);
+
+#endif /* _LINUX_JITALLOC_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index 4751031f3f..ff26a4f0c9 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1202,6 +1202,9 @@ config LRU_GEN_STATS
This option has a per-memcg and per-node memory overhead.
# }

+config JITALLOC
+ bool
+
source "mm/damon/Kconfig"

endmenu
diff --git a/mm/Makefile b/mm/Makefile
index c03e1e5859..25e82db9e8 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -138,3 +138,4 @@ obj-$(CONFIG_IO_MAPPING) += io-mapping.o
obj-$(CONFIG_HAVE_BOOTMEM_INFO_NODE) += bootmem_info.o
obj-$(CONFIG_GENERIC_IOREMAP) += ioremap.o
obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
+obj-$(CONFIG_JITALLOC) += jitalloc.o
diff --git a/mm/jitalloc.c b/mm/jitalloc.c
new file mode 100644
index 0000000000..7c4d621802
--- /dev/null
+++ b/mm/jitalloc.c
@@ -0,0 +1,187 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/gfp.h>
+#include <linux/highmem.h>
+#include <linux/jitalloc.h>
+#include <linux/mm.h>
+#include <linux/moduleloader.h>
+#include <linux/mutex.h>
+#include <linux/set_memory.h>
+#include <linux/vmalloc.h>
+
+#include <asm/text-patching.h>
+
+static DEFINE_MUTEX(jit_alloc_lock);
+
+struct jit_cache {
+ unsigned obj_size_bits;
+ unsigned objs_per_slab;
+ struct list_head partial;
+};
+
+#define JITALLOC_MIN_SIZE 16
+#define NR_JIT_CACHES ilog2(PAGE_SIZE / JITALLOC_MIN_SIZE)
+
+static struct jit_cache jit_caches[NR_JIT_CACHES];
+
+struct jit_slab {
+ unsigned long __page_flags;
+
+ struct jit_cache *cache;
+ void *executably_mapped;;
+ unsigned long *objs_allocated; /* bitmap of free objects */
+ struct list_head list;
+};
+
+#define folio_jit_slab(folio) (_Generic((folio), \
+ const struct folio *: (const struct jit_slab *)(folio), \
+ struct folio *: (struct jit_slab *)(folio)))
+
+#define jit_slab_folio(s) (_Generic((s), \
+ const struct jit_slab *: (const struct folio *)s, \
+ struct jit_slab *: (struct folio *)s))
+
+static struct jit_slab *jit_slab_alloc(struct jit_cache *cache)
+{
+ void *executably_mapped = module_alloc(PAGE_SIZE);
+ struct page *page;
+ struct folio *folio;
+ struct jit_slab *slab;
+ unsigned long *objs_allocated;
+
+ if (!executably_mapped)
+ return NULL;
+
+ objs_allocated = kcalloc(BITS_TO_LONGS(cache->objs_per_slab), sizeof(unsigned long), GFP_KERNEL);
+ if (!objs_allocated ) {
+ vfree(executably_mapped);
+ return NULL;
+ }
+
+ set_vm_flush_reset_perms(executably_mapped);
+ set_memory_rox((unsigned long) executably_mapped, 1);
+
+ page = vmalloc_to_page(executably_mapped);
+ folio = page_folio(page);
+
+ __folio_set_slab(folio);
+ slab = folio_jit_slab(folio);
+ slab->cache = cache;
+ slab->executably_mapped = executably_mapped;
+ slab->objs_allocated = objs_allocated;
+ INIT_LIST_HEAD(&slab->list);
+
+ return slab;
+}
+
+static void *jit_cache_alloc(void *buf, size_t len, struct jit_cache *cache)
+{
+ struct jit_slab *s =
+ list_first_entry_or_null(&cache->partial, struct jit_slab, list) ?:
+ jit_slab_alloc(cache);
+ unsigned obj_idx, nr_allocated;
+
+ if (!s)
+ return NULL;
+
+ obj_idx = find_first_zero_bit(s->objs_allocated, cache->objs_per_slab);
+
+ BUG_ON(obj_idx >= cache->objs_per_slab);
+ __set_bit(obj_idx, s->objs_allocated);
+
+ nr_allocated = bitmap_weight(s->objs_allocated, s->cache->objs_per_slab);
+
+ if (nr_allocated == s->cache->objs_per_slab) {
+ list_del_init(&s->list);
+ } else if (nr_allocated == 1) {
+ list_del(&s->list);
+ list_add(&s->list, &s->cache->partial);
+ }
+
+ return s->executably_mapped + (obj_idx << cache->obj_size_bits);
+}
+
+void jit_update(void *buf, void *new_buf, size_t len)
+{
+ text_poke_copy(buf, new_buf, len);
+}
+EXPORT_SYMBOL_GPL(jit_update);
+
+void jit_free(void *buf)
+{
+ struct page *page;
+ struct folio *folio;
+ struct jit_slab *s;
+ unsigned obj_idx, nr_allocated;
+ size_t offset;
+
+ if (!buf)
+ return;
+
+ page = vmalloc_to_page(buf);
+ folio = page_folio(page);
+ offset = offset_in_folio(folio, buf);
+
+ if (!folio_test_slab(folio)) {
+ vfree(buf);
+ return;
+ }
+
+ s = folio_jit_slab(folio);
+
+ mutex_lock(&jit_alloc_lock);
+ obj_idx = offset >> s->cache->obj_size_bits;
+
+ __clear_bit(obj_idx, s->objs_allocated);
+
+ nr_allocated = bitmap_weight(s->objs_allocated, s->cache->objs_per_slab);
+
+ if (nr_allocated == 0) {
+ list_del(&s->list);
+ kfree(s->objs_allocated);
+ folio_put(folio);
+ } else if (nr_allocated + 1 == s->cache->objs_per_slab) {
+ list_del(&s->list);
+ list_add(&s->list, &s->cache->partial);
+ }
+
+ mutex_unlock(&jit_alloc_lock);
+}
+EXPORT_SYMBOL_GPL(jit_free);
+
+void *jit_alloc(void *buf, size_t len)
+{
+ unsigned jit_cache_idx = ilog2(roundup_pow_of_two(len) / 16);
+ void *p;
+
+ if (jit_cache_idx < NR_JIT_CACHES) {
+ mutex_lock(&jit_alloc_lock);
+ p = jit_cache_alloc(buf, len, &jit_caches[jit_cache_idx]);
+ mutex_unlock(&jit_alloc_lock);
+ } else {
+ p = module_alloc(len);
+ if (p) {
+ set_vm_flush_reset_perms(p);
+ set_memory_rox((unsigned long) p, DIV_ROUND_UP(len, PAGE_SIZE));
+ }
+ }
+
+ if (p && buf)
+ jit_update(p, buf, len);
+
+ return p;
+}
+EXPORT_SYMBOL_GPL(jit_alloc);
+
+static int __init jit_alloc_init(void)
+{
+ for (unsigned i = 0; i < ARRAY_SIZE(jit_caches); i++) {
+ jit_caches[i].obj_size_bits = ilog2(JITALLOC_MIN_SIZE) + i;
+ jit_caches[i].objs_per_slab = PAGE_SIZE >> jit_caches[i].obj_size_bits;
+
+ INIT_LIST_HEAD(&jit_caches[i].partial);
+ }
+
+ return 0;
+}
+core_initcall(jit_alloc_init);