Messages in this thread | | | Subject | Re: [PATCH 1/3] iommu/vt-d: skip RMRR entries that fail the sanity check | From | "Chen, Yian" <> | Date | Tue, 17 Dec 2019 11:19:28 -0800 |
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On 12/16/2019 11:35 AM, Barret Rhoden wrote: > On 12/16/19 2:07 PM, Chen, Yian wrote: >> >> >> On 12/11/2019 11:46 AM, Barret Rhoden wrote: >>> RMRR entries describe memory regions that are DMA targets for devices >>> outside the kernel's control. >>> >>> RMRR entries that fail the sanity check are pointing to regions of >>> memory that the firmware did not tell the kernel are reserved or >>> otherwise should not be used. >>> >>> Instead of aborting DMAR processing, this commit skips these RMRR >>> entries. They will not be mapped into the IOMMU, but the IOMMU can >>> still be utilized. If anything, when the IOMMU is on, those devices >>> will not be able to clobber RAM that the kernel has allocated from >>> those >>> regions. >>> >>> Signed-off-by: Barret Rhoden <brho@google.com> >>> --- >>> drivers/iommu/intel-iommu.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c >>> index f168cd8ee570..f7e09244c9e4 100644 >>> --- a/drivers/iommu/intel-iommu.c >>> +++ b/drivers/iommu/intel-iommu.c >>> @@ -4316,7 +4316,7 @@ int __init dmar_parse_one_rmrr(struct >>> acpi_dmar_header *header, void *arg) >>> rmrr = (struct acpi_dmar_reserved_memory *)header; >>> ret = arch_rmrr_sanity_check(rmrr); >>> if (ret) >>> - return ret; >>> + return 0; >>> rmrru = kzalloc(sizeof(*rmrru), GFP_KERNEL); >>> if (!rmrru) >> Parsing rmrr function should report the error to caller. The behavior >> to response the error can be >> chose by the caller in the calling stack, for example, >> dmar_walk_remapping_entries(). >> A concern is that ignoring a detected firmware bug might have a >> potential side impact though >> it seemed safe for your case. > > That's a little difficult given the current code. Once we are in > dmar_walk_remapping_entries(), the specific function (parse_one_rmrr) > is called via callback: > > ret = cb->cb[iter->type](iter, cb->arg[iter->type]); > if (ret) > return ret; > > If there's an error of any sort, it aborts the walk. Handling the > specific errors here is difficult, since we don't know what the errors > mean to the specific callback. Is there some errno we can use that > means "there was a problem, but it's not so bad that you have to > abort, but I figured you ought to know"? Not that I think that's a > good idea. > > The knowledge of whether or not a specific error is worth aborting all > DMAR functionality is best known inside the specific callback. The > only handling to do is print a warning and either skip it or abort. > > I think skipping the entry for a bad RMRR is better than aborting > completely, though I understand if people don't like that. It's > debatable. By aborting, we lose the ability to use the IOMMU at all, > but we are still in a situation where the devices using the RMRR > regions might be clobbering kernel memory, right? Using the IOMMU > (with no mappings for the bad RMRRs) would stop those devices from > clobbering memory. > > Regardless, I have two other patches in this series that could resolve > the problem for me and probably other people. I'd just like at least > one of the three patches to get merged so that my machine boots when > the original commit f036c7fa0ab6 ("iommu/vt-d: Check VT-d RMRR region > in BIOS is reported as reserved") gets released. > when a firmware bug appears, the potential problem may beyond the scope of its visible impacts so that introducing a workaround in official implementation should be considered very carefully.
If the workaround is really needed at this point, I would recommend adding a WARN_TAINT with TAINT_FIRMWARE_WORKAROUND, to tell the workaround is in the place.
Thanks Yian
> Thanks, > > Barret >
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