archive mirror
 help / color / mirror / Atom feed
From: Dave Hansen <>
To: Jarkko Sakkinen <>,
Cc: Haitao Huang <>,
	Dave Hansen <>,
	Thomas Gleixner <>,
	Ingo Molnar <>, Borislav Petkov <>,, "H. Peter Anvin" <>,
	Jethro Beekman <>,
	Sean Christopherson <>,
Subject: Re: [PATCH 2/2] x86/sgx: Maintain encl->refcount for each encl->mm_list entry
Date: Fri, 5 Feb 2021 11:36:57 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

[-- Attachment #1: Type: text/plain, Size: 1213 bytes --]

On 2/5/21 10:28 AM, Jarkko Sakkinen wrote:
> This has been shown in tests:
> [  +0.000008] WARNING: CPU: 3 PID: 7620 at kernel/rcu/srcutree.c:374 cleanup_srcu_struct+0xed/0x100
> There are two functions that drain encl->mm_list:
> - sgx_release() (i.e. VFS release) removes the remaining mm_list entries.
> - sgx_mmu_notifier_release() removes mm_list entry for the registered
>   process, if it still exists.

Jarkko, I like your approach.  This actually has the potential to be a
lot more understandable than the fix we settled on before.

But I think the explanation needs some tweaking, and I think I can take
it a step further to make it even more straightforward.  The issue here
isn't *really* mm_list, it's this:

	encl_mm->encl = encl;

That literally establishes a encl_mm to encl reference and needs a
reference count.  That reference remains until 'encl_mm' is freed.  I
don't think mm_list needs to even be taken into account.

The most straightforward way to fix this is to take a refcount at
"encl_mm->encl = encl" and release it at kfree(encl_mm).  That makes a
*lot* of logical sense to me, and it's also trivial to audit.

Totally untested patch attached (adapted directly from yours).

[-- Attachment #2: raw.patch --]
[-- Type: text/x-patch, Size: 3004 bytes --]

This has been shown in tests:

[  +0.000008] WARNING: CPU: 3 PID: 7620 at kernel/rcu/srcutree.c:374 cleanup_srcu_struct+0xed/0x100

This is essentially a use-after free, although SRCU notices it as
an SRCU cleanup in an invalid context.

== Background ==

SGX has a data structure (struct sgx_encl_mm) which keeps per-mm SGX
metadata.  This is separate from 'struct sgx_encl' because, in theory,
an enclave can be mapped from more than one mm.  sgx_encl_mm includes
a pointer back to the sgx_encl.

This means that sgx_encl must have a longer lifetime than all of the
sgx_encl_mm's that point to it.  That's usually the case: sgx_encl_mm
is freed only after the mmu_notifier is unregistered in sgx_release().

However, there's a race.  If the process is exiting,
sgx_mmu_notifier_release() can be called in parallel with sgx_release()
instead of being called *by* it.  The mmu_notifier path keeps encl_mm
alive past when sgx_encl can be freed.  This inverts the lifetime rules
and means that sgx_mmu_notifier_release() can access a freed sgx_encl.

== Fix ==

Increase encl->refcount when encl_mm->encl is established. Release
this reference encl_mm is freed.  This ensures that 'encl' outlives

Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Cc: Dave Hansen <
Reported-by: Haitao Huang <>
Signed-off-by: Jarkko Sakkinen <>

 b/arch/x86/kernel/cpu/sgx/driver.c |    3 +++
 b/arch/x86/kernel/cpu/sgx/encl.c   |    5 +++++
 2 files changed, 8 insertions(+)

diff -puN arch/x86/kernel/cpu/sgx/driver.c~raw arch/x86/kernel/cpu/sgx/driver.c
--- a/arch/x86/kernel/cpu/sgx/driver.c~raw	2021-02-05 10:52:47.484545869 -0800
+++ b/arch/x86/kernel/cpu/sgx/driver.c	2021-02-05 10:59:06.497544923 -0800
@@ -72,6 +72,9 @@ static int sgx_release(struct inode *ino
 		mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
+		/* 'encl_mm' is gone, put encl_mm->encl reference: */
+		kref_put(&encl->refcount, sgx_encl_release);
 	kref_put(&encl->refcount, sgx_encl_release);
diff -puN arch/x86/kernel/cpu/sgx/encl.c~raw arch/x86/kernel/cpu/sgx/encl.c
--- a/arch/x86/kernel/cpu/sgx/encl.c~raw	2021-02-05 10:52:47.486545869 -0800
+++ b/arch/x86/kernel/cpu/sgx/encl.c	2021-02-05 11:23:06.674541332 -0800
@@ -481,6 +481,9 @@ static void sgx_mmu_notifier_free(struct
 	struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm, mmu_notifier);
+	/* 'encl_mm' is goin away, put encl_mm->encl reference: */
+	kref_put(&encl_mm->encl->refcount, sgx_encl_release);
@@ -534,6 +537,8 @@ int sgx_encl_mm_add(struct sgx_encl *enc
 	if (!encl_mm)
 		return -ENOMEM;
+	/* Grab a refcount for the encl_mm->encl reference: */
+	kref_get(&encl->refcount);
 	encl_mm->encl = encl;
 	encl_mm->mm = mm;
 	encl_mm->mmu_notifier.ops = &sgx_mmu_notifier_ops;

  reply	other threads:[~2021-02-05 19:40 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-05 18:28 [PATCH 1/2] MAINTAINERS: Add Dave Hansen as reviewer for INTEL SGX Jarkko Sakkinen
2021-02-05 18:28 ` [PATCH 2/2] x86/sgx: Maintain encl->refcount for each encl->mm_list entry Jarkko Sakkinen
2021-02-05 19:36   ` Dave Hansen [this message]
2021-02-07 21:29     ` Jarkko Sakkinen
2021-02-07 21:32       ` Jarkko Sakkinen
     [not found] <>
2021-02-05 15:15 ` Jarkko Sakkinen

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).