Re: [syzbot] general protection fault in skb_dequeue (3)

From: David Howells
Date: Thu Feb 02 2023 - 10:16:12 EST

David Hildenbrand <david@xxxxxxxxxx> wrote:

> At first, I wondered if that's related to shared anonymous pages getting
> pinned R/O that would trigger COW-unsharing ... but I don't even see where we
> are supposed to use FOLL_PIN vs. FOLL_GET here? IOW, we're not even supposed
> to access user space memory (neither FOLL_GET nor FOLL_PIN) but still end up
> with a change in behavior.

I'm not sure that using FOLL_PIN is part of the problem here.

sendfile() is creating a transient buffer attached to a pipe, doing a DIO read
into it (which uses iov_iter_extract_pages() to populate a bio) and then feeds
the pages from the pipe one at a time using a BVEC-type iterator to a buffered
write.

Note that iov_iter_extract_pages() does not pin/get pages when accessing a
BVEC, KVEC, XARRAY or PIPE iterator. However, in this case, this should not
matter as the pipe is holding refs on the pages in the buffer.

I have found that the issue goes away if I add an extra get_page() into
iov_iter_extract_pipe_pages() and don't release it (the page is then leaked).

This makes me think that the problem might be due to the pages getting
recycled from the pipe before DIO has finished writing to them - but that
shouldn't be the case as the splice has to be synchronous - we can't mark data
in the pipe as 'produced' until we've finished reading it.

I've also turned on CONFIG_DEBUG_PAGE_REF with a twist of my own to use a page
flag to mark the pages of interest and only trace those pages (see attached
patch) - and that consistently shows the pages being released after the bio is
cleared.

Further, when it does arise, the issue only happens when two or more copies of
the test program are run in parallel - which makes me wonder if truncate is
implicated, except that that would have to happen inside the filesystem and be
nothing to do with the pipe in the middle of sendfile().

David
--
commit 8ec9d7d951166d77e283079151b496632c290958
Author: David Howells <dhowells@xxxxxxxxxx>
Date: Fri Nov 13 13:21:01 2020 +0000

mm: Add a page bit to trigger page_ref tracing

Add a facility to mark a page using an extra page bit so that the page is
flagged in tracing. Also make it possible to restrict the tracepoint to
only marked pages. The mark is automatically removed when the page is
freed.

Enable the followng:

CONFIG_DEBUG_PAGE_REF
CONFIG_DEBUG_PAGE_MARK
CONFIG_DEBUG_PAGE_REF_ONLY_MARKED

and then enable the page_ref tracepoints:

echo 1 >/sys/kernel/debug/tracing/events/page_ref/enable

and it shows the fate of pages created by readahead for the netfs helpers.

This also contains some bits to turn it on. If you want to trace a
filesystem other than XFS, you need to change the magic numbers in the
patch.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>

diff --git a/block/bio.c b/block/bio.c
index fc57f0aa098e..69b62ebf68ed 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -20,6 +20,7 @@
#include <linux/blk-crypto.h>
#include <linux/xarray.h>

+#include <trace/events/page_ref.h>
#include <trace/events/block.h>
#include "blk.h"
#include "blk-rq-qos.h"
@@ -1174,10 +1175,17 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
struct bio_vec *bvec;

bio_for_each_segment_all(bvec, bio, iter_all) {
- if (mark_dirty && !PageCompound(bvec->bv_page))
- set_page_dirty_lock(bvec->bv_page);
- bio_release_page(bio, bvec->bv_page);
+ if (PageDebugMark(bvec->bv_page))
+ trace_page_ref_set(bvec->bv_page, 999);
}
+
+ if (bio_flagged(bio, BIO_PAGE_REFFED) ||
+ bio_flagged(bio, BIO_PAGE_PINNED))
+ bio_for_each_segment_all(bvec, bio, iter_all) {
+ if (mark_dirty && !PageCompound(bvec->bv_page))
+ set_page_dirty_lock(bvec->bv_page);
+ bio_release_page(bio, bvec->bv_page);
+ }
}
EXPORT_SYMBOL_GPL(__bio_release_pages);

diff --git a/fs/splice.c b/fs/splice.c
index 5969b7a1d353..4c2f13f5d6d5 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -304,6 +304,7 @@ ssize_t generic_file_splice_read(struct file *in, loff_t *ppos,
int ret;

iov_iter_pipe(&to, ITER_DEST, pipe, len);
+ to.debug = true;
init_sync_kiocb(&kiocb, in);
kiocb.ki_pos = *ppos;
ret = call_read_iter(in, &kiocb, &to);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index b2c09997d79c..cafa26637067 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -484,8 +484,8 @@ void zero_fill_bio(struct bio *bio);

static inline void bio_release_pages(struct bio *bio, bool mark_dirty)
{
- if (bio_flagged(bio, BIO_PAGE_REFFED) ||
- bio_flagged(bio, BIO_PAGE_PINNED))
+ //if (bio_flagged(bio, BIO_PAGE_REFFED) ||
+ // bio_flagged(bio, BIO_PAGE_PINNED))
__bio_release_pages(bio, mark_dirty);
}

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 69e93a0c1277..80cbf784239e 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -138,6 +138,9 @@ enum pageflags {
#endif
#ifdef CONFIG_KASAN_HW_TAGS
PG_skip_kasan_poison,
+#endif
+#ifdef CONFIG_DEBUG_PAGE_MARK
+ PG_debug_mark,
#endif
__NR_PAGEFLAGS,

@@ -694,6 +697,15 @@ static __always_inline bool PageKsm(struct page *page)
TESTPAGEFLAG_FALSE(Ksm, ksm)
#endif

+#ifdef CONFIG_DEBUG_PAGE_MARK
+/*
+ * Debug marks are just used for page_ref tracepoint control and display.
+ */
+PAGEFLAG(DebugMark, debug_mark, PF_ANY)
+#else
+TESTPAGEFLAG_FALSE(DebugMark, debug_mark)
+#endif
+
u64 stable_page_flags(struct page *page);

/**
diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index d7c2d33baa7f..7bc1a94d9cbb 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -24,7 +24,11 @@ DECLARE_TRACEPOINT(page_ref_unfreeze);
*
* See trace_##name##_enabled(void) in include/linux/tracepoint.h
*/
-#define page_ref_tracepoint_active(t) tracepoint_enabled(t)
+#ifndef CONFIG_DEBUG_PAGE_REF_ONLY_MARKED
+#define page_ref_tracepoint_active(p, t) tracepoint_enabled(t)
+#else
+#define page_ref_tracepoint_active(p, t) (tracepoint_enabled(t) && PageDebugMark(p))
+#endif

extern void __page_ref_set(struct page *page, int v);
extern void __page_ref_mod(struct page *page, int v);
@@ -36,7 +40,7 @@ extern void __page_ref_unfreeze(struct page *page, int v);

#else

-#define page_ref_tracepoint_active(t) false
+#define page_ref_tracepoint_active(page, t) false

static inline void __page_ref_set(struct page *page, int v)
{
@@ -97,7 +101,7 @@ static inline int page_count(const struct page *page)
static inline void set_page_count(struct page *page, int v)
{
atomic_set(&page->_refcount, v);
- if (page_ref_tracepoint_active(page_ref_set))
+ if (page_ref_tracepoint_active(page, page_ref_set))
__page_ref_set(page, v);
}

@@ -118,7 +122,7 @@ static inline void init_page_count(struct page *page)
static inline void page_ref_add(struct page *page, int nr)
{
atomic_add(nr, &page->_refcount);
- if (page_ref_tracepoint_active(page_ref_mod))
+ if (page_ref_tracepoint_active(page, page_ref_mod))
__page_ref_mod(page, nr);
}

@@ -130,7 +134,7 @@ static inline void folio_ref_add(struct folio *folio, int nr)
static inline void page_ref_sub(struct page *page, int nr)
{
atomic_sub(nr, &page->_refcount);
- if (page_ref_tracepoint_active(page_ref_mod))
+ if (page_ref_tracepoint_active(page, page_ref_mod))
__page_ref_mod(page, -nr);
}

@@ -143,7 +147,7 @@ static inline int page_ref_sub_return(struct page *page, int nr)
{
int ret = atomic_sub_return(nr, &page->_refcount);

- if (page_ref_tracepoint_active(page_ref_mod_and_return))
+ if (page_ref_tracepoint_active(page, page_ref_mod_and_return))
__page_ref_mod_and_return(page, -nr, ret);
return ret;
}
@@ -156,7 +160,7 @@ static inline int folio_ref_sub_return(struct folio *folio, int nr)
static inline void page_ref_inc(struct page *page)
{
atomic_inc(&page->_refcount);
- if (page_ref_tracepoint_active(page_ref_mod))
+ if (page_ref_tracepoint_active(page, page_ref_mod))
__page_ref_mod(page, 1);
}

@@ -168,7 +172,7 @@ static inline void folio_ref_inc(struct folio *folio)
static inline void page_ref_dec(struct page *page)
{
atomic_dec(&page->_refcount);
- if (page_ref_tracepoint_active(page_ref_mod))
+ if (page_ref_tracepoint_active(page, page_ref_mod))
__page_ref_mod(page, -1);
}

@@ -181,7 +185,7 @@ static inline int page_ref_sub_and_test(struct page *page, int nr)
{
int ret = atomic_sub_and_test(nr, &page->_refcount);

- if (page_ref_tracepoint_active(page_ref_mod_and_test))
+ if (page_ref_tracepoint_active(page, page_ref_mod_and_test))
__page_ref_mod_and_test(page, -nr, ret);
return ret;
}
@@ -195,7 +199,7 @@ static inline int page_ref_inc_return(struct page *page)
{
int ret = atomic_inc_return(&page->_refcount);

- if (page_ref_tracepoint_active(page_ref_mod_and_return))
+ if (page_ref_tracepoint_active(page, page_ref_mod_and_return))
__page_ref_mod_and_return(page, 1, ret);
return ret;
}
@@ -209,7 +213,7 @@ static inline int page_ref_dec_and_test(struct page *page)
{
int ret = atomic_dec_and_test(&page->_refcount);

- if (page_ref_tracepoint_active(page_ref_mod_and_test))
+ if (page_ref_tracepoint_active(page, page_ref_mod_and_test))
__page_ref_mod_and_test(page, -1, ret);
return ret;
}
@@ -223,7 +227,7 @@ static inline int page_ref_dec_return(struct page *page)
{
int ret = atomic_dec_return(&page->_refcount);

- if (page_ref_tracepoint_active(page_ref_mod_and_return))
+ if (page_ref_tracepoint_active(page, page_ref_mod_and_return))
__page_ref_mod_and_return(page, -1, ret);
return ret;
}
@@ -237,7 +241,7 @@ static inline bool page_ref_add_unless(struct page *page, int nr, int u)
{
bool ret = atomic_add_unless(&page->_refcount, nr, u);

- if (page_ref_tracepoint_active(page_ref_mod_unless))
+ if (page_ref_tracepoint_active(page, page_ref_mod_unless))
__page_ref_mod_unless(page, nr, ret);
return ret;
}
@@ -317,7 +321,7 @@ static inline int page_ref_freeze(struct page *page, int count)
{
int ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count);

- if (page_ref_tracepoint_active(page_ref_freeze))
+ if (page_ref_tracepoint_active(page, page_ref_freeze))
__page_ref_freeze(page, count, ret);
return ret;
}
@@ -333,7 +337,7 @@ static inline void page_ref_unfreeze(struct page *page, int count)
VM_BUG_ON(count == 0);

atomic_set_release(&page->_refcount, count);
- if (page_ref_tracepoint_active(page_ref_unfreeze))
+ if (page_ref_tracepoint_active(page, page_ref_unfreeze))
__page_ref_unfreeze(page, count);
}

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 514e3b7b06b8..89272c05d74d 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -45,6 +45,7 @@ struct iov_iter {
bool nofault;
bool data_source;
bool user_backed;
+ bool debug;
union {
size_t iov_offset;
int last_offset;
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index 412b5a46374c..5f3b9b0e4b53 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -103,6 +103,12 @@
#define IF_HAVE_PG_SKIP_KASAN_POISON(flag,string)
#endif

+#ifdef CONFIG_DEBUG_PAGE_MARK
+#define IF_HAVE_PG_DEBUG_MARK(flag,string) ,{1UL << flag, string}
+#else
+#define IF_HAVE_PG_DEBUG_MARK(flag,string)
+#endif
+
#define __def_pageflag_names \
{1UL << PG_locked, "locked" }, \
{1UL << PG_waiters, "waiters" }, \
@@ -132,7 +138,8 @@ IF_HAVE_PG_IDLE(PG_young, "young" ) \
IF_HAVE_PG_IDLE(PG_idle, "idle" ) \
IF_HAVE_PG_ARCH_X(PG_arch_2, "arch_2" ) \
IF_HAVE_PG_ARCH_X(PG_arch_3, "arch_3" ) \
-IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
+IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison") \
+IF_HAVE_PG_DEBUG_MARK(PG_debug_mark, "debug_mark" )

#define show_page_flags(flags) \
(flags) ? __print_flags(flags, "|", \
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index d69a05950555..7ac030208a2c 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -10,9 +10,11 @@
#include <linux/vmalloc.h>
#include <linux/splice.h>
#include <linux/compat.h>
+#include <linux/page-flags.h>
#include <net/checksum.h>
#include <linux/scatterlist.h>
#include <linux/instrumented.h>
+#include <trace/events/page_ref.h>

#define PIPE_PARANOIA /* for now */

@@ -1331,6 +1333,10 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
struct page *page = append_pipe(i, left, &off);
if (!page)
break;
+ if (i->debug && !PageDebugMark(page)) {
+ //SetPageDebugMark(page);
+ get_page(page);
+ }
chunk = min_t(size_t, left, PAGE_SIZE - off);
get_page(*p++ = page);
}
@@ -1955,6 +1961,11 @@ static ssize_t iov_iter_extract_pipe_pages(struct iov_iter *i,
struct page *page = append_pipe(i, left, &offset);
if (!page)
break;
+ if (i->debug && !PageDebugMark(page)) {
+ SetPageDebugMark(page);
+ trace_page_ref_set(page, 888);
+ //get_page(page);
+ }
chunk = min_t(size_t, left, PAGE_SIZE - offset);
left -= chunk;
*p++ = page;
diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index fca699ad1fb0..111a946a676f 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -149,6 +149,23 @@ config DEBUG_PAGE_REF
kernel code. However the runtime performance overhead is virtually
nil until the tracepoints are actually enabled.

+config DEBUG_PAGE_MARK
+ bool "Reserve a page bit to mark pages to be debugged"
+ depends on DEBUG_PAGE_REF
+ help
+ This option adds an extra page flag that can be used to mark pages
+ for debugging. The mark can be observed in the page_ref tracepoints.
+ The mark isn't set on any pages without alteration of the code. This
+ is intended for filesystem debugging and code to set the mark must be
+ added manually into the source.
+
+config DEBUG_PAGE_REF_ONLY_MARKED
+ bool "Only trace marked pages"
+ depends on DEBUG_PAGE_REF && DEBUG_PAGE_MARK
+ help
+ This option restricts the page_ref tracepoints to only track marked
+ pages.
+
config DEBUG_RODATA_TEST
bool "Testcase for the marking rodata read-only"
depends on STRICT_KERNEL_RWX
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0745aedebb37..37f146e5b2eb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1102,6 +1102,9 @@ static inline void __free_one_page(struct page *page,

VM_BUG_ON(!zone_is_initialized(zone));
VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
+#ifdef CONFIG_DEBUG_PAGE_MARK
+ ClearPageDebugMark(page);
+#endif

VM_BUG_ON(migratetype == -1);
if (likely(!is_migrate_isolate(migratetype)))
diff --git a/mm/readahead.c b/mm/readahead.c
index b10f0cf81d80..c5558daf3a56 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -248,6 +248,10 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
folio = filemap_alloc_folio(gfp_mask, 0);
if (!folio)
break;
+#define XFS_SUPER_MAGIC 0x58465342 /* "XFSB" */
+ if (mapping->host->i_sb->s_magic == XFS_SUPER_MAGIC)
+ folio_set_debug_mark(folio);
+
if (filemap_add_folio(mapping, folio, index + i,
gfp_mask) < 0) {
folio_put(folio);
@@ -809,6 +813,7 @@ void readahead_expand(struct readahead_control *ractl,
page = __page_cache_alloc(gfp_mask);
if (!page)
return;
+ //SetPageDebugMark(page);
if (add_to_page_cache_lru(page, mapping, index, gfp_mask) < 0) {
put_page(page);
return;
@@ -832,6 +837,7 @@ void readahead_expand(struct readahead_control *ractl,
page = __page_cache_alloc(gfp_mask);
if (!page)
return;
+ //SetPageDebugMark(page);
if (add_to_page_cache_lru(page, mapping, index, gfp_mask) < 0) {
put_page(page);
return;