Re: [PATCH] iommu/amd: Check PM_LEVEL_SIZE() condition in locked section

From: Jerry Snitselaar
Date: Fri Oct 18 2019 - 10:48:13 EST

On Fri Oct 18 19, Joerg Roedel wrote:
On Thu, Oct 17, 2019 at 07:36:51AM -0400, Qian Cai wrote:

> On Oct 16, 2019, at 6:59 PM, Jerry Snitselaar <jsnitsel@xxxxxxxxxx> wrote:
> I guess the mode level 6 check is really for other potential callers
> increase_address_space, none exist at the moment, and the condition
> of the while loop in alloc_pte should fail if the mode level is 6.

Because there is no locking around iommu_map_page(), if there are
several concurrent callers of it for the same domain, could it be that
it silently corrupt data due to invalid access?

No, that can't happen because increase_address_space locks the domain
before actually doing anything. So the address space can't grow above
domain->mode == 6. But what can happen is that the WARN_ON_ONCE triggers
in there and that the address space is increased multiple times when
only one increase would be sufficient.

To fix this we just need to check the PM_LEVEL_SIZE() condition again
when we hold the lock:

From e930e792a998e89dfd4feef15fbbf289c45124dc Mon Sep 17 00:00:00 2001
From: Joerg Roedel <jroedel@xxxxxxx>
Date: Fri, 18 Oct 2019 11:34:22 +0200
Subject: [PATCH] iommu/amd: Check PM_LEVEL_SIZE() condition in locked section

The increase_address_space() function has to check the PM_LEVEL_SIZE()
condition again under the domain->lock to avoid a false trigger of the
WARN_ON_ONCE() and to avoid that the address space is increase more
often than necessary.

Reported-by: Qian Cai <cai@xxxxxx>
Fixes: 754265bcab78 ("iommu/amd: Fix race in increase_address_space()")
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
drivers/iommu/amd_iommu.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
index 2369b8af81f3..a0639e511ffe 100644
--- a/drivers/iommu/amd_iommu.c
+++ b/drivers/iommu/amd_iommu.c
@@ -1463,6 +1463,7 @@ static void free_pagetable(struct protection_domain *domain)
* to 64 bits.
static bool increase_address_space(struct protection_domain *domain,
+ unsigned long address,
gfp_t gfp)
unsigned long flags;
@@ -1471,8 +1472,8 @@ static bool increase_address_space(struct protection_domain *domain,

spin_lock_irqsave(&domain->lock, flags);

- if (WARN_ON_ONCE(domain->mode == PAGE_MODE_6_LEVEL))
- /* address space already 64 bit large */
+ if (address <= PM_LEVEL_SIZE(domain->mode) ||
+ WARN_ON_ONCE(domain->mode == PAGE_MODE_6_LEVEL))
goto out;

pte = (void *)get_zeroed_page(gfp);
@@ -1505,7 +1506,7 @@ static u64 *alloc_pte(struct protection_domain *domain,

while (address > PM_LEVEL_SIZE(domain->mode))
- *updated = increase_address_space(domain, gfp) || *updated;
+ *updated = increase_address_space(domain, address, gfp) || *updated;

level = domain->mode - 1;
pte = &domain->pt_root[PM_LEVEL_INDEX(level, address)];

Reviewed-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>