[PATCH] iommu/amd: Use array_index_nospec() for rlookup_table index

[PATCH] iommu/amd: Use array_index_nospec() for rlookup_table index

[Dheeraj Kumar Srivastava]

Use array_index_nospec() to prevent speculative out-of-bounds
access when indexing pci_seg->rlookup_table with a user provided
device id.

Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Reviewed-by: Sairaj Kodilkar <Sairaj.ArunKodilkar@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202510281233.q4cBnp3z-lkp@intel.com/
---
 drivers/iommu/amd/debugfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
index 10fa217a7119..4990f6db99ef 100644
--- a/drivers/iommu/amd/debugfs.c
+++ b/drivers/iommu/amd/debugfs.c
@@ -174,6 +174,7 @@ static ssize_t devid_write(struct file *filp, const char __user *ubuf,
 			kfree(srcid_ptr);
 			return -EINVAL;
 		}
+		devid = array_index_nospec(devid, (u32)pci_seg->last_bdf + 1);
 		iommu = pci_seg->rlookup_table[devid];
 		if (!iommu) {
 			kfree(srcid_ptr);
-- 
2.25.1

Re: [PATCH] iommu/amd: Use array_index_nospec() for rlookup_table index

[Dan Carpenter]

On Fri, Jan 09, 2026 at 10:50:40AM +0530, Dheeraj Kumar Srivastava wrote:
> Use array_index_nospec() to prevent speculative out-of-bounds
> access when indexing pci_seg->rlookup_table with a user provided
> device id.
> 
> Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
> Reviewed-by: Sairaj Kodilkar <Sairaj.ArunKodilkar@amd.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Dan Carpenter <error27@gmail.com>
> Closes: https://lore.kernel.org/r/202510281233.q4cBnp3z-lkp@intel.com/

This is interesting because more and more people are using lei to
recieve email and now they get unfiltered Smatch warnings from zero day
bot.

Normally, I just ignore these warnings because they're hard to review
and I recently modified Smatch to stop the zero day bot from warning
about them.

The problem is that I've tried to contact people from Intel to help
review some of the warnings but I've never recieved a response.  I've
heard that Intel has a handful of people that deal with Spectre v1 bugs
but I've never seen any evidence of that...  I've never tried reaching
out to AMD.

> ---
>  drivers/iommu/amd/debugfs.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
> index 10fa217a7119..4990f6db99ef 100644
> --- a/drivers/iommu/amd/debugfs.c
> +++ b/drivers/iommu/amd/debugfs.c
> @@ -174,6 +174,7 @@ static ssize_t devid_write(struct file *filp, const char __user *ubuf,
>  			kfree(srcid_ptr);
>  			return -EINVAL;
>  		}
> +		devid = array_index_nospec(devid, (u32)pci_seg->last_bdf + 1);

This is debugfs so it's already root only.  The cast to (u32) is
unnecessary.

regards,
dan carpenter

>  		iommu = pci_seg->rlookup_table[devid];
>  		if (!iommu) {
>  			kfree(srcid_ptr);
> -- 
> 2.25.1

Re: [PATCH] iommu/amd: Use array_index_nospec() for rlookup_table index

[Dheeraj Kumar Srivastava]

Hi Dan,

On 1/9/2026 11:35 AM, Dan Carpenter wrote:
> On Fri, Jan 09, 2026 at 10:50:40AM +0530, Dheeraj Kumar Srivastava wrote:
>> Use array_index_nospec() to prevent speculative out-of-bounds
>> access when indexing pci_seg->rlookup_table with a user provided
>> device id.
>>
>> Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
>> Reviewed-by: Sairaj Kodilkar <Sairaj.ArunKodilkar@amd.com>
>> Reported-by: kernel test robot <lkp@intel.com>
>> Reported-by: Dan Carpenter <error27@gmail.com>
>> Closes: https://lore.kernel.org/r/202510281233.q4cBnp3z-lkp@intel.com/
> 
> This is interesting because more and more people are using lei to
> recieve email and now they get unfiltered Smatch warnings from zero day
> bot.
> 
> Normally, I just ignore these warnings because they're hard to review
> and I recently modified Smatch to stop the zero day bot from warning
> about them.
> 
> The problem is that I've tried to contact people from Intel to help
> review some of the warnings but I've never recieved a response.  I've
> heard that Intel has a handful of people that deal with Spectre v1 bugs
> but I've never seen any evidence of that...  I've never tried reaching
> out to AMD.
> 
>> ---
>>   drivers/iommu/amd/debugfs.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
>> index 10fa217a7119..4990f6db99ef 100644
>> --- a/drivers/iommu/amd/debugfs.c
>> +++ b/drivers/iommu/amd/debugfs.c
>> @@ -174,6 +174,7 @@ static ssize_t devid_write(struct file *filp, const char __user *ubuf,
>>   			kfree(srcid_ptr);
>>   			return -EINVAL;
>>   		}
>> +		devid = array_index_nospec(devid, (u32)pci_seg->last_bdf + 1);
> 
> This is debugfs so it's already root only.  The cast to (u32) is
> unnecessary.
> 

I agree that the (u32) cast is unnecessary here and will remove it.

When you mentioned that this is debugfs and therefore root-only, could 
you clarify the context of that comment? I just want to make sure I’m 
interpreting the rationale correctly.

Thanks
Dheeraj

> regards,
> dan carpenter
> 
>>   		iommu = pci_seg->rlookup_table[devid];
>>   		if (!iommu) {
>>   			kfree(srcid_ptr);
>> -- 
>> 2.25.1

Re: [PATCH] iommu/amd: Use array_index_nospec() for rlookup_table index

[Dan Carpenter]

On Fri, Jan 09, 2026 at 02:19:03PM +0530, Dheeraj Kumar Srivastava wrote:
> Hi Dan,
> 
> On 1/9/2026 11:35 AM, Dan Carpenter wrote:
> > On Fri, Jan 09, 2026 at 10:50:40AM +0530, Dheeraj Kumar Srivastava wrote:
> > > Use array_index_nospec() to prevent speculative out-of-bounds
> > > access when indexing pci_seg->rlookup_table with a user provided
> > > device id.
> > > 
> > > Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
> > > Reviewed-by: Sairaj Kodilkar <Sairaj.ArunKodilkar@amd.com>
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > Reported-by: Dan Carpenter <error27@gmail.com>
> > > Closes: https://lore.kernel.org/r/202510281233.q4cBnp3z-lkp@intel.com/
> > 
> > This is interesting because more and more people are using lei to
> > recieve email and now they get unfiltered Smatch warnings from zero day
> > bot.
> > 
> > Normally, I just ignore these warnings because they're hard to review
> > and I recently modified Smatch to stop the zero day bot from warning
> > about them.
> > 
> > The problem is that I've tried to contact people from Intel to help
> > review some of the warnings but I've never recieved a response.  I've
> > heard that Intel has a handful of people that deal with Spectre v1 bugs
> > but I've never seen any evidence of that...  I've never tried reaching
> > out to AMD.
> > 
> > > ---
> > >   drivers/iommu/amd/debugfs.c | 1 +
> > >   1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
> > > index 10fa217a7119..4990f6db99ef 100644
> > > --- a/drivers/iommu/amd/debugfs.c
> > > +++ b/drivers/iommu/amd/debugfs.c
> > > @@ -174,6 +174,7 @@ static ssize_t devid_write(struct file *filp, const char __user *ubuf,
> > >   			kfree(srcid_ptr);
> > >   			return -EINVAL;
> > >   		}
> > > +		devid = array_index_nospec(devid, (u32)pci_seg->last_bdf + 1);
> > 
> > This is debugfs so it's already root only.  The cast to (u32) is
> > unnecessary.
> > 
> 
> I agree that the (u32) cast is unnecessary here and will remove it.
> 
> When you mentioned that this is debugfs and therefore root-only, could you
> clarify the context of that comment? I just want to make sure I’m
> interpreting the rationale correctly.
> 

Debugfs is always read only unless you are root.  Anything else is insane.
So it's like sure, this patch is correct from a correctness point of view
but in terms of is this exploitable?  No.

regards,
dan carpenter