| Message ID | 20250206234234.1912585-1-balbirs@nvidia.com (mailing list archive) |
|---|---|
| State | New |
| Delegated to: | Bjorn Helgaas |
| Headers | show |
| Series | [v2] x86/kaslr: Revisit entropy when CONFIG_PCI_P2PDMA is enabled | expand |
On 2/6/25 15:42, Balbir Singh wrote: > Fix this by not shrinking the size of direct map when CONFIG_PCI_P2PDMA > is enabled. This reduces the total available entropy, but it's > better than the current work around of having to disable KASLR > completely. Is the size of these P2PDMA mappings known up front? Or do you just need them to be as large as possible?
On Fri, Feb 07, 2025 at 10:42:34AM +1100, Balbir Singh wrote: > When CONFIG_PCI_P2PDMA is enabled, it maps the PFN's via a > ZONE_DEVICE mapping using devm_memremap_pages(). The mapped > virtual address range corresponds to the pci_resource_start() > of the BAR address and size corresponding to the BAR length. > > When KASLR is enabled, the direct map range of the kernel is > reduced to the size of physical memory plus additional padding. > If the BAR address is beyond this limit, PCI peer to peer DMA > mappings fail. > > Fix this by not shrinking the size of direct map when CONFIG_PCI_P2PDMA > is enabled. This reduces the total available entropy, but it's > better than the current work around of having to disable KASLR > completely. > > Cc: Dave Hansen <dave.hansen@linux.intel.com> > Cc: Andy Lutomirski <luto@kernel.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Borislav Petkov <bp@alien8.de> > Cc: "H. Peter Anvin" <hpa@zytor.com> > Cc: Kees Cook <kees@kernel.org> > Cc: Bjorn Helgaas <bhelgaas@google.com> > Link: https://lore.kernel.org/lkml/20250206023201.1481957-1-balbirs@nvidia.com/ > > Signed-off-by: Balbir Singh <balbirs@nvidia.com> Thanks for the update! Reviewed-by: Kees Cook <kees@kernel.org>
On 2/7/25 11:27, Dave Hansen wrote: > On 2/6/25 15:42, Balbir Singh wrote: >> Fix this by not shrinking the size of direct map when CONFIG_PCI_P2PDMA >> is enabled. This reduces the total available entropy, but it's >> better than the current work around of having to disable KASLR >> completely. > > Is the size of these P2PDMA mappings known up front? Or do you just need > them to be as large as possible? The size is not known upfront, it depends on the system configuration. Yes, we need them to be as large as possible. Balbir
On 2/7/25 12:00, Kees Cook wrote: > On Fri, Feb 07, 2025 at 10:42:34AM +1100, Balbir Singh wrote: >> When CONFIG_PCI_P2PDMA is enabled, it maps the PFN's via a >> ZONE_DEVICE mapping using devm_memremap_pages(). The mapped >> virtual address range corresponds to the pci_resource_start() >> of the BAR address and size corresponding to the BAR length. >> >> When KASLR is enabled, the direct map range of the kernel is >> reduced to the size of physical memory plus additional padding. >> If the BAR address is beyond this limit, PCI peer to peer DMA >> mappings fail. >> >> Fix this by not shrinking the size of direct map when CONFIG_PCI_P2PDMA >> is enabled. This reduces the total available entropy, but it's >> better than the current work around of having to disable KASLR >> completely. >> >> Cc: Dave Hansen <dave.hansen@linux.intel.com> >> Cc: Andy Lutomirski <luto@kernel.org> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: Ingo Molnar <mingo@redhat.com> >> Cc: Borislav Petkov <bp@alien8.de> >> Cc: "H. Peter Anvin" <hpa@zytor.com> >> Cc: Kees Cook <kees@kernel.org> >> Cc: Bjorn Helgaas <bhelgaas@google.com> >> Link: https://lore.kernel.org/lkml/20250206023201.1481957-1-balbirs@nvidia.com/ >> >> Signed-off-by: Balbir Singh <balbirs@nvidia.com> > > Thanks for the update! > > Reviewed-by: Kees Cook <kees@kernel.org> > Thank you Kees! I wanted to request that we pick this up in linux-next via the relevant subtree for further testing Balbir Singh
diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c index 11a93542d198..3c306de52fd4 100644 --- a/arch/x86/mm/kaslr.c +++ b/arch/x86/mm/kaslr.c @@ -113,8 +113,14 @@ void __init kernel_randomize_memory(void) memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) + CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING; - /* Adapt physical memory region size based on available memory */ - if (memory_tb < kaslr_regions[0].size_tb) + /* + * Adapt physical memory region size based on available memory, + * except when CONFIG_PCI_P2PDMA is enabled. P2PDMA exposes the + * device BAR space assuming the direct map space is large enough + * for creating a ZONE_DEVICE mapping in the direct map corresponding + * to the physical BAR address. + */ + if (!IS_ENABLED(CONFIG_PCI_P2PDMA) && (memory_tb < kaslr_regions[0].size_tb)) kaslr_regions[0].size_tb = memory_tb; /* diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig index 2fbd379923fd..5c3054aaec8c 100644 --- a/drivers/pci/Kconfig +++ b/drivers/pci/Kconfig @@ -203,6 +203,12 @@ config PCI_P2PDMA P2P DMA transactions must be between devices behind the same root port. + Enabling this option will reduce the entropy of x86 KASLR memory + regions. For example - on a 46 bit system, the entropy goes down + from 16 bits to 15 bits. The actual reduction in entropy depends + on the physical address bits, on processor features, kernel config + (5 level page table) and physical memory present on the system. + If unsure, say N. config PCI_LABEL
When CONFIG_PCI_P2PDMA is enabled, it maps the PFN's via a ZONE_DEVICE mapping using devm_memremap_pages(). The mapped virtual address range corresponds to the pci_resource_start() of the BAR address and size corresponding to the BAR length. When KASLR is enabled, the direct map range of the kernel is reduced to the size of physical memory plus additional padding. If the BAR address is beyond this limit, PCI peer to peer DMA mappings fail. Fix this by not shrinking the size of direct map when CONFIG_PCI_P2PDMA is enabled. This reduces the total available entropy, but it's better than the current work around of having to disable KASLR completely. Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@redhat.com> Cc: Borislav Petkov <bp@alien8.de> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Kees Cook <kees@kernel.org> Cc: Bjorn Helgaas <bhelgaas@google.com> Link: https://lore.kernel.org/lkml/20250206023201.1481957-1-balbirs@nvidia.com/ Signed-off-by: Balbir Singh <balbirs@nvidia.com> --- Changelog v2 - Add information about entropy drop when PCI_P2PDMA is selected Testing: commit 0483e1fa6e09d ("x86/mm: Implement ASLR for kernel memory regions") mentions the problems that the following problems need to be addressed. 1 The three target memory sections are never at the same place between boots. 2 The physical memory mapping can use a virtual address not aligned on the PGD page table. 3 Have good entropy early at boot before get_random_bytes is available. 4 Add optional padding for memory hotplug compatibility. Ran an automated test to ensure that (1) holds true across several iterations of automated reboot testing. 2, 3 and 4 are not impacted by this patch. Manual Testing on a system where the problem reproduces 1. With KASLR Hotplug memory [0x240000000000-0x242000000000] exceeds maximum addressable range [0x0-0xaffffffffff] ------------[ cut here ]------------ 2. With the fixes added peer-to-peer DMA memory 0x240000000000-0x241fffffffff KASLR is still enabled as seen by kaslr_offset() (difference between __START_KERNEL and _stext) 3. Without the fixes and KASLR disabled arch/x86/mm/kaslr.c | 10 ++++++++-- drivers/pci/Kconfig | 6 ++++++ 2 files changed, 14 insertions(+), 2 deletions(-)