[PATCH v8] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

[PATCH v8] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

[Jarkko Sakkinen]

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
'encl_mm'.

Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Cc: Dave Hansen <dave.hansen@linux.intel.com
Reported-by: Haitao Huang <haitao.huang@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v8:
- Slight adjustments on call sites suggested by Dave, to make things
  more clear and obvious. Otherwise, semantically same as v7:
  https://lore.kernel.org/linux-sgx/b874673d-9d58-0d6f-ce2d-ef4d33ac5115@intel.com/
  Contains also long description written by Dave.
v7:
- No changes from v6. Resend of
  https://patchwork.kernel.org/project/intel-sgx/patch/20210204143845.39697-1-jarkko@kernel.org/
v6:
- Maintain refcount for each encl->mm_list entry.
v5:
- To make sure that the instance does not get deleted use kref_get()
  kref_put(). This also removes the need for additional
  synchronize_srcu().
v4:
- Rewrite the commit message.
- Just change the call order. *_expedited() is out of scope for this
  bug fix.
v3: Fine-tuned tags, and added missing change log for v2.
v2: Switch to synchronize_srcu_expedited().

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

diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c
index f2eac41bb4ff..8ce6d8371cfb 100644
--- a/arch/x86/kernel/cpu/sgx/driver.c
+++ b/arch/x86/kernel/cpu/sgx/driver.c
@@ -72,6 +72,9 @@ static int sgx_release(struct inode *inode, struct file *file)
 		synchronize_srcu(&encl->srcu);
 		mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
 		kfree(encl_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 --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 20a2dd5ba2b4..7449ef33f081 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -473,6 +473,9 @@ static void sgx_mmu_notifier_free(struct mmu_notifier *mn)
 {
 	struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm, mmu_notifier);
 
+	/* 'encl_mm' is going away, put encl_mm->encl reference: */
+	kref_put(&encl_mm->encl->refcount, sgx_encl_release);
+
 	kfree(encl_mm);
 }
 
@@ -526,6 +529,8 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm)
 	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;
-- 
2.30.0

Re: [PATCH v8] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

[Dave Hansen]

> 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.
...

Acked-by: Dave Hansen <dave.hansen@linux.intel.com

[tip: x86/urgent] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

[tip-bot2 for Jarkko Sakkinen]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     2ade0d60939bcd54197c133b03b460fe62a4ec47
Gitweb:        https://git.kernel.org/tip/2ade0d60939bcd54197c133b03b460fe62a4ec47
Author:        Jarkko Sakkinen <jarkko@kernel.org>
AuthorDate:    Mon, 08 Feb 2021 00:14:01 +02:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Mon, 08 Feb 2021 19:11:30 +01:00

x86/sgx: Maintain encl->refcount for each encl->mm_list entry

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 when encl_mm is freed. This ensures that encl outlives
encl_mm.

 [ bp: Massage commit message. ]

Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Reported-by: Haitao Huang <haitao.huang@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20210207221401.29933-1-jarkko@kernel.org
---
 arch/x86/kernel/cpu/sgx/driver.c | 3 +++
 arch/x86/kernel/cpu/sgx/encl.c   | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c
index f2eac41..8ce6d83 100644
--- a/arch/x86/kernel/cpu/sgx/driver.c
+++ b/arch/x86/kernel/cpu/sgx/driver.c
@@ -72,6 +72,9 @@ static int sgx_release(struct inode *inode, struct file *file)
 		synchronize_srcu(&encl->srcu);
 		mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
 		kfree(encl_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 --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index ee50a50..f65564a 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -481,6 +481,9 @@ static void sgx_mmu_notifier_free(struct mmu_notifier *mn)
 {
 	struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm, mmu_notifier);
 
+	/* 'encl_mm' is going away, put encl_mm->encl reference: */
+	kref_put(&encl_mm->encl->refcount, sgx_encl_release);
+
 	kfree(encl_mm);
 }
 
@@ -534,6 +537,8 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm)
 	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;

Re: [PATCH v8] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

[Haitao Huang]

On Sun, 07 Feb 2021 16:14:01 -0600, Jarkko Sakkinen <jarkko@kernel.org>  
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
>
> This is essentially a use-after free, although SRCU notices it as
> an SRCU cleanup in an invalid context.
>
The comments in code around this warning indicate a potential memory leak.
Not sure how use-after-free come into play. Anyway, this fix seems to work  
for the warning above.

However, I still have doubts on another potential race. See below.


> diff --git a/arch/x86/kernel/cpu/sgx/driver.c  
> b/arch/x86/kernel/cpu/sgx/driver.c
> index f2eac41bb4ff..8ce6d8371cfb 100644
> --- a/arch/x86/kernel/cpu/sgx/driver.c
> +++ b/arch/x86/kernel/cpu/sgx/driver.c
> @@ -72,6 +72,9 @@ static int sgx_release(struct inode *inode, struct  
> file *file)
>  		synchronize_srcu(&encl->srcu);
>  		mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
>  		kfree(encl_mm);

Note here you are freeing the encl_mm, outside protection of  
encl->refcount.

> +
> +		/* 'encl_mm' is gone, put encl_mm->encl reference: */
> +		kref_put(&encl->refcount, sgx_encl_release);
>  	}
> 	kref_put(&encl->refcount, sgx_encl_release);
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c  
> b/arch/x86/kernel/cpu/sgx/encl.c
> index 20a2dd5ba2b4..7449ef33f081 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -473,6 +473,9 @@ static void sgx_mmu_notifier_free(struct  
> mmu_notifier *mn)
>  {
>  	struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm,  
> mmu_notifier);
> +	/* 'encl_mm' is going away, put encl_mm->encl reference: */
> +	kref_put(&encl_mm->encl->refcount, sgx_encl_release);
> +
>  	kfree(encl_mm);

Could this access to and kfree of encl_mm possibly be after the  
kfree(encl_mm) noted above?

Also is there a reason we do kfree(encl_mm) in notifier_free not directly  
in notifier_release?

Thanks
Haitao

Re: [PATCH v8] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

[Sean Christopherson]

On Tue, Apr 13, 2021, Haitao Huang wrote:
> On Sun, 07 Feb 2021 16:14:01 -0600, Jarkko Sakkinen <jarkko@kernel.org>
> 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
> > 
> > This is essentially a use-after free, although SRCU notices it as
> > an SRCU cleanup in an invalid context.
> > 
> The comments in code around this warning indicate a potential memory leak.
> Not sure how use-after-free come into play. Anyway, this fix seems to work
> for the warning above.
> 
> However, I still have doubts on another potential race. See below.
> 
> 
> > diff --git a/arch/x86/kernel/cpu/sgx/driver.c
> > b/arch/x86/kernel/cpu/sgx/driver.c
> > index f2eac41bb4ff..8ce6d8371cfb 100644
> > --- a/arch/x86/kernel/cpu/sgx/driver.c
> > +++ b/arch/x86/kernel/cpu/sgx/driver.c
> > @@ -72,6 +72,9 @@ static int sgx_release(struct inode *inode, struct
> > file *file)
> >  		synchronize_srcu(&encl->srcu);
> >  		mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
> >  		kfree(encl_mm);
> 
> Note here you are freeing the encl_mm, outside protection of encl->refcount.
> 
> > +
> > +		/* 'encl_mm' is gone, put encl_mm->encl reference: */
> > +		kref_put(&encl->refcount, sgx_encl_release);
> >  	}
> > 	kref_put(&encl->refcount, sgx_encl_release);
> > diff --git a/arch/x86/kernel/cpu/sgx/encl.c
> > b/arch/x86/kernel/cpu/sgx/encl.c
> > index 20a2dd5ba2b4..7449ef33f081 100644
> > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > @@ -473,6 +473,9 @@ static void sgx_mmu_notifier_free(struct
> > mmu_notifier *mn)
> >  {
> >  	struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm,
> > mmu_notifier);
> > +	/* 'encl_mm' is going away, put encl_mm->encl reference: */
> > +	kref_put(&encl_mm->encl->refcount, sgx_encl_release);
> > +
> >  	kfree(encl_mm);
> 
> Could this access to and kfree of encl_mm possibly be after the
> kfree(encl_mm) noted above?

No, the mmu_notifier_unregister() ensures that all in-progress notifiers complete
before it returns, i.e. SGX's notifier call back is not reachable after it's
unregistered.

> Also is there a reason we do kfree(encl_mm) in notifier_free not directly in
> notifier_release?

Because encl_mm is the anchor to the enclave reference

	/* 'encl_mm' is going away, put encl_mm->encl reference: */
	kref_put(&encl_mm->encl->refcount, sgx_encl_release);

as well as the mmu notifier reference (the mmu_notifier_put(mn) call chain).
Freeing encl_mm immediately would prevent sgx_mmu_notifier_free() from dropping
the enclave reference.  And the mmu notifier reference need to be dropped in
sgx_mmu_notifier_release() because the encl_mm has been taken off encl->mm_list.

Re: [PATCH v8] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

[Dave Hansen]

On 4/14/21 8:51 AM, Sean Christopherson wrote:
>> Could this access to and kfree of encl_mm possibly be after the
>> kfree(encl_mm) noted above?
> No, the mmu_notifier_unregister() ensures that all in-progress notifiers complete
> before it returns, i.e. SGX's notifier call back is not reachable after it's
> unregistered.
> 
>> Also is there a reason we do kfree(encl_mm) in notifier_free not directly in
>> notifier_release?
> Because encl_mm is the anchor to the enclave reference
> 
> 	/* 'encl_mm' is going away, put encl_mm->encl reference: */
> 	kref_put(&encl_mm->encl->refcount, sgx_encl_release);
> 
> as well as the mmu notifier reference (the mmu_notifier_put(mn) call chain).
> Freeing encl_mm immediately would prevent sgx_mmu_notifier_free() from dropping
> the enclave reference.  And the mmu notifier reference need to be dropped in
> sgx_mmu_notifier_release() because the encl_mm has been taken off encl->mm_list.

Haitao, I think you've highlighted that this locking scheme is woefully
under-documented.  Any patches to beef it up would be very welcome.