Re: d_lookup: Unable to handle kernel paging request

Al Viro
2019-06-18T18:35:53Z

On Tue, May 28, 2019 at 11:38:43AM +0200, Vicente Bergas wrote:
> On Wednesday, May 22, 2019 6:29:46 PM CEST, Al Viro wrote:
> > ...
> > IOW, here we have also run into bogus hlist forward pointer or head -
> > same 0x1000000 in one case and 0x0000880001000000 in two others.
> >
> > Have you tried to see if KASAN catches anything on those loads?
> > Use-after-free, for example... Another thing to try: slap
> > WARN_ON(entry->d_flags & DCACHE_NORCU);
> > in __d_rehash() and see if it triggers.
>
> Hi Al,
> after 5 days with v5.2-rc1 + KASAN + WARN_ON could not reproduce the issue.
> Neither the first day running v5.3-rc2 + WARN_ON. But today 6 times.
> So, there is no KASAN and also the WARN_ON, being there, did not trigger.
> The first trace hapenned while untaring a big file into tmpfs. The other
> five while "git pull -r" severeal repos on f2fs.
>
> Regards,
> Vicenç.
>

__d_lookup() running into &dentry->d_hash == 0x01000000 at some point in hash chain
and trying to look at ->d_name.hash:

> pc : __d_lookup+0x58/0x198
> lr : d_lookup+0x38/0x68
> sp : ffff000012663b90
> x29: ffff000012663b90 x28: ffff000012663d58 x27: 0000000000000000 x26:
> ffff8000ae7cc900 x25: 0000000000000001 x24: ffffffffffffffff x23:
> 00000000ce9c8f81 x22: 0000000000000000 x21: 0000000000000001 x20:
> ffff000012663d58 x19: 0000000001000000 x18: 0000000000000000 x17:
> 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14:
> 0000000000000000 x13: 0000000000000000 x12: 0000000000000000 x11:
> fefefefefefefeff x10: b4fea3d0a3a4b4fe x9 : d237122a91454b69 x8 :
> a0591ae4450bed6a x7 : 5845a2c80f79d4e7 x6 : 0000000000000004 x5 :
> 0000000000000000 x4 : ffff000012663d58 x3 : ffff000010828a68 x2 :
> ffff000010828000 x1 : ffff8000f3000000 x0 : 00000000000674e4 Call trace:

__d_lookup_rcu() running into &dentry->d_hash == 0x01000000 at some point in hash
chain and trying to look at ->d_seq:

> pc : __d_lookup_rcu+0x68/0x198
> lr : lookup_fast+0x44/0x2e8
> sp : ffff0000130b3b60
> x29: ffff0000130b3b60 x28: 00000000ce99d070 x27: ffffffffffffffff x26:
> 0000000000000026 x25: ffff8000ecec6030 x24: ffff0000130b3c2c x23:
> 0000000000000006 x22: 00000026ce99d070 x21: ffff8000811f3d80 x20:
> 0000000000020000 x19: 0000000001000000 x18: 0000000000000000 x17:
> 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14:
> 0000000000000000 x13: 0000000000000000 x12: 0000000000000000 x11:
> fefefefefefefeff x10: e4d0b2e6e2b4b6e9 x9 : 5096e90463dfacb0 x8 :
> 2b4f8961c30ebc93 x7 : aec349fb204a7256 x6 : 4fd9025392b5761a x5 :
> 02ff010101030100 x4 : ffff8000f3000000 x3 : ffff0000130b3d58 x2 :
> ffff0000130b3c2c x1 : 00000000000674ce x0 : ffff8000811f3d80 Call trace:

__d_lookup_rcu() running into &dentry->d_hash == 0x0000880001000000 at some point
in hash chain and trying to look at ->d_seq:

> pc : __d_lookup_rcu+0x68/0x198
> lr : lookup_fast+0x44/0x2e8
> sp : ffff00001325ba90
> x29: ffff00001325ba90 x28: 00000000ce99f075 x27: ffffffffffffffff x26:
> 0000000000000007 x25: ffff8000ecec402a x24: ffff00001325bb5c x23:
> 0000000000000007 x22: 00000007ce99f075 x21: ffff80007a810c00 x20:
> 0000000000000000 x19: 0000880001000000 x18: 0000000000000000 x17:
> 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14:
> 0000000000000000 x13: 0000000000000000 x12: 0000000000000000 x11:
> fefefefefefefeff x10: d0bbbcbfa6b2b9bc x9 : 0000000000000000 x8 :
> ffff80007a810c00 x7 : 6cad9ff29d8de19c x6 : ff94ec6f0ce3656c x5 :
> ffff8000ecec402a x4 : ffff8000f3000000 x3 : ffff00001325bc78 x2 :
> ffff00001325bb5c x1 : 00000000000674cf x0 : ffff80007a810c00 Call trace:

ditto

> pc : __d_lookup_rcu+0x68/0x198
> lr : lookup_fast+0x44/0x2e8
> sp : ffff000012a3ba90
> x29: ffff000012a3ba90 x28: 00000000ce99f075 x27: ffffffffffffffff x26:
> 0000000000000007 x25: ffff8000ecec702a x24: ffff000012a3bb5c x23:
> 0000000000000007 x22: 00000007ce99f075 x21: ffff80007a810c00 x20:
> 0000000000000000 x19: 0000880001000000 x18: 0000000000000000 x17:
> 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14:
> 0000000000000000 x13: 0000000000000000 x12: 0000000000000000 x11:
> fefefefefefefeff x10: d0bbbcbfa6b2b9bc x9 : 0000000000000000 x8 :
> ffff80007a810c00 x7 : 6cad9ff29d8de19c x6 : ff94ec6f0ce3656c x5 :
> ffff8000ecec702a x4 : ffff8000f3000000 x3 : ffff000012a3bc78 x2 :
> ffff000012a3bb5c x1 : 00000000000674cf x0 : ffff80007a810c00 Call trace:

ditto

> pc : __d_lookup_rcu+0x68/0x198
> lr : lookup_fast+0x44/0x2e8
> sp : ffff0000132bba90
> x29: ffff0000132bba90 x28: 00000000ce99e1a6 x27: ffffffffffffffff x26:
> 000000000000000c x25: ffff8000f21dd036 x24: ffff0000132bbb5c x23:
> 0000000000000004 x22: 0000000cce99e1a6 x21: ffff800074dd8d80 x20:
> 0000000000000000 x19: 0000880001000000 x18: 0000000000000000 x17:
> 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14:
> 0000000000000000 x13: 0000000000000000 x12: 0000000000000000 x11:
> fefefefefefefeff x10: d0d0d0d0b8fea4b3 x9 : 40bcd8645005512e x8 :
> c433ade89ebd10f9 x7 : c6b69091eeb194d2 x6 : 848f758ca69635b4 x5 :
> ffff8000f21dd036 x4 : ffff8000f3000000 x3 : ffff0000132bbc78 x2 :
> ffff0000132bbb5c x1 : 00000000000674cf x0 : ffff800074dd8d80 Call trace:

... and ditto:

> pc : __d_lookup_rcu+0x68/0x198
> lr : lookup_fast+0x44/0x2e8
> sp : ffff000013263a90
> x29: ffff000013263a90 x28: 00000000ce99e1a6 x27: ffffffffffffffff x26:
> 000000000000000c x25: ffff8000f0a6f036 x24: ffff000013263b5c x23:
> 0000000000000004 x22: 0000000cce99e1a6 x21: ffff800074dd8d80 x20:
> 0000000000000000 x19: 0000880001000000 x18: 0000000000000000 x17:
> 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14:
> 0000000000000000 x13: 0000000000000000 x12: 0000000000000000 x11:
> fefefefefefefeff x10: d0d0d0d0b8fea4b3 x9 : 40bcd8645005512e x8 :
> c433ade89ebd10f9 x7 : c6b69091eeb194d2 x6 : 848f758ca69635b4 x5 :
> ffff8000f0a6f036 x4 : ffff8000f3000000 x3 : ffff000013263c78 x2 :
> ffff000013263b5c x1 : 00000000000674cf x0 : ffff800074dd8d80 Call trace:

All of those run under rcu_read_lock() and no dentry with DCACHE_NORCU has
ever been inserted into a hash chain, so it doesn't look like a plain
use-after-free. Could you try something like the following to see a bit
more about where it comes from?

So far it looks like something is buggering a forward reference
in hash chain in a fairly specific way - the values seen had been
00000000010000000 and
00008800010000000. Does that smell like anything from arm64-specific
data structures (PTE, etc.)?

Alternatively, we might've gone off rails a step (or more) before,
with the previous iteration going through bogus, but at least mapped
address - the one that has never been a dentry in the first place.

diff --git a/fs/dcache.c b/fs/dcache.c
index c435398f2c81..cb555edb5b55 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2114,6 +2114,22 @@ static inline bool d_same_name(const struct dentry *dentry,
name) == 0;
}

+static void dump(struct dentry *dentry)
+{
+ int i;
+ if (!dentry) {
+ printk(KERN_ERR "list fucked in head");
+ return;
+ }
+ printk(KERN_ERR "fucked dentry[%p]: d_hash.next = %p, flags = %x, count = %d",
+ dentry, dentry->d_hash.next, dentry->d_flags,
+ dentry->d_lockref.count
+ );
+ for (i = 0; i < sizeof(struct dentry); i++)
+ printk(KERN_CONT "%c%02x", i & 31 ? ' ' : '\n',
+ ((unsigned char *)dentry)[i]);
+}
+
/**
* __d_lookup_rcu - search for a dentry (racy, store-free)
* @parent: parent dentry
@@ -2151,7 +2167,7 @@ struct dentry *__d_lookup_rcu(const struct dentry *parent,
const unsigned char *str = name->name;
struct hlist_bl_head *b = d_hash(hashlen_hash(hashlen));
struct hlist_bl_node *node;
- struct dentry *dentry;
+ struct dentry *dentry, *last = NULL;

/*
* Note: There is significant duplication with __d_lookup_rcu which is
@@ -2176,6 +2192,10 @@ struct dentry *__d_lookup_rcu(const struct dentry *parent,
hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
unsigned seq;

+ if (unlikely((u32)(unsigned long)&dentry->d_hash == 0x01000000))
+ dump(last);
+ last = dentry;
+
seqretry:
/*
* The dentry sequence count protects us from concurrent
@@ -2274,7 +2294,7 @@ struct dentry *__d_lookup(const struct dentry *parent, const struct qstr *name)
struct hlist_bl_head *b = d_hash(hash);
struct hlist_bl_node *node;
struct dentry *found = NULL;
- struct dentry *dentry;
+ struct dentry *dentry, *last = NULL;

/*
* Note: There is significant duplication with __d_lookup_rcu which is
@@ -2300,6 +2320,10 @@ struct dentry *__d_lookup(const struct dentry *parent, const struct qstr *name)

hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {

+ if (unlikely((u32)(unsigned long)&dentry->d_hash == 0x01000000))
+ dump(last);
+ last = dentry;
+
if (dentry->d_name.hash != hash)
continue;