Dan Williams
2021-Aug-25 00:58 UTC
[PATCH v2 1/3] /dev/mem: disallow access to explicitly excluded system RAM regions
On Mon, Aug 16, 2021 at 7:25 AM David Hildenbrand <david at redhat.com> wrote:> > virtio-mem dynamically exposes memory inside a device memory region as > system RAM to Linux, coordinating with the hypervisor which parts are > actually "plugged" and consequently usable/accessible. On the one hand, the > virtio-mem driver adds/removes whole memory blocks, creating/removing busy > IORESOURCE_SYSTEM_RAM resources, on the other hand, it logically (un)plugs > memory inside added memory blocks, dynamically either exposing them to > the buddy or hiding them from the buddy and marking them PG_offline. > > virtio-mem wants to make sure that in a sane environment, nobody > "accidentially" accesses unplugged memory inside the device managed > region. After /proc/kcore has been sanitized and /dev/kmem has been > removed, /dev/mem is the remaining interface that still allows uncontrolled > access to the device-managed region of virtio-mem devices from user > space. > > There is no known sane use case for mapping virtio-mem device memory > via /dev/mem while virtio-mem driver concurrently (un)plugs memory inside > that region. So once the driver was loaded and detected the device > along the device-managed region, we just want to disallow any access via > /dev/mem to it. > > Let's add the basic infrastructure to exclude some physical memory > regions completely from /dev/mem access, on any architecture and under > any system configuration (independent of CONFIG_STRICT_DEVMEM and > independent of "iomem=").I'm certainly on team "/dev/mem considered harmful", but this approach feels awkward. It feels wrong for being non-committal about whether CONFIG_STRICT_DEVMEM is in wide enough use that the safety can be turned on all the time, and the configuration option dropped, or there are users clinging onto /dev/mem where they expect to be able to build a debug kernel to turn all of these restrictions off, even the virtio-mem ones. This splits the difference and says some /dev/mem accesses are always disallowed for "reasons", but I could say the same thing about pmem, there's no sane reason to allow /dev/mem which has no idea about the responsibilities of properly touching pmem to get access to it.> > Any range marked with "IORESOURCE_SYSTEM_RAM | IORESOURCE_EXCLUSIVE" > will be excluded, even if not busy. For now, there are no applicable > ranges and we'll modify virtio-mem next to properly set > IORESOURCE_EXCLUSIVE on the parent resource. > > As next_resource() will iterate over children although we might want to > skip a certain range completely, let's add and use > next_range_skip_children() and for_each_resource(), to optimize that case, > avoding having to traverse subtrees that are not of interest. > > Signed-off-by: David Hildenbrand <david at redhat.com> > --- > drivers/char/mem.c | 22 ++++++++------------ > include/linux/ioport.h | 1 + > kernel/resource.c | 47 ++++++++++++++++++++++++++++++++++++++++++ > lib/Kconfig.debug | 4 +++- > 4 files changed, 60 insertions(+), 14 deletions(-) > > diff --git a/drivers/char/mem.c b/drivers/char/mem.c > index 1c596b5cdb27..1829dc6a1f29 100644 > --- a/drivers/char/mem.c > +++ b/drivers/char/mem.c > @@ -60,13 +60,18 @@ static inline int valid_mmap_phys_addr_range(unsigned long pfn, size_t size) > } > #endif > > -#ifdef CONFIG_STRICT_DEVMEM > static inline int page_is_allowed(unsigned long pfn) > { > - return devmem_is_allowed(pfn); > +#ifdef CONFIG_STRICT_DEVMEM > + if (!devmem_is_allowed(pfn)) > + return 0; > +#endif /* CONFIG_STRICT_DEVMEM */ > + return !iomem_range_contains_excluded_devmem(PFN_PHYS(pfn), PAGE_SIZE); > } > + > static inline int range_is_allowed(unsigned long pfn, unsigned long size) > { > +#ifdef CONFIG_STRICT_DEVMEM > u64 from = ((u64)pfn) << PAGE_SHIFT; > u64 to = from + size; > u64 cursor = from; > @@ -77,18 +82,9 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size) > cursor += PAGE_SIZE; > pfn++; > } > - return 1; > -} > -#else > -static inline int page_is_allowed(unsigned long pfn) > -{ > - return 1; > -} > -static inline int range_is_allowed(unsigned long pfn, unsigned long size) > -{ > - return 1; > +#endif /* CONFIG_STRICT_DEVMEM */ > + return !iomem_range_contains_excluded_devmem(PFN_PHYS(pfn), size); > } > -#endif > > #ifndef unxlate_dev_mem_ptr > #define unxlate_dev_mem_ptr unxlate_dev_mem_ptr > diff --git a/include/linux/ioport.h b/include/linux/ioport.h > index 8359c50f9988..d31f83281327 100644 > --- a/include/linux/ioport.h > +++ b/include/linux/ioport.h > @@ -308,6 +308,7 @@ extern struct resource * __devm_request_region(struct device *dev, > extern void __devm_release_region(struct device *dev, struct resource *parent, > resource_size_t start, resource_size_t n); > extern int iomem_map_sanity_check(resource_size_t addr, unsigned long size); > +extern bool iomem_range_contains_excluded_devmem(u64 addr, u64 size); > extern bool iomem_is_exclusive(u64 addr); > > extern int > diff --git a/kernel/resource.c b/kernel/resource.c > index ca9f5198a01f..f57a14617c49 100644 > --- a/kernel/resource.c > +++ b/kernel/resource.c > @@ -73,6 +73,18 @@ static struct resource *next_resource(struct resource *p) > return p->sibling; > } > > +static struct resource *next_resource_skip_children(struct resource *p) > +{ > + while (!p->sibling && p->parent) > + p = p->parent; > + return p->sibling; > +} > + > +#define for_each_resource(_root, _p, _skip_children) \ > + for ((_p) = (_root)->child; (_p); \ > + (_p) = (_skip_children) ? next_resource_skip_children(_p) : \ > + next_resource(_p)) > + > static void *r_next(struct seq_file *m, void *v, loff_t *pos) > { > struct resource *p = v; > @@ -1700,6 +1712,41 @@ int iomem_map_sanity_check(resource_size_t addr, unsigned long size) > return err; > } > > +/* > + * Check if a physical memory range is completely excluded from getting > + * mapped/accessed via /dev/mem. > + */ > +bool iomem_range_contains_excluded_devmem(u64 addr, u64 size) > +{ > + const unsigned int flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_EXCLUSIVE; > + bool skip_children = false, excluded = false; > + struct resource *p; > + > + read_lock(&resource_lock); > + for_each_resource(&iomem_resource, p, skip_children) { > + if (p->start >= addr + size) > + break; > + if (p->end < addr) { > + skip_children = true; > + continue; > + } > + skip_children = false; > + > + /* > + * A system RAM resource is excluded if IORESOURCE_EXCLUSIVE > + * is set, even if not busy and even if we don't have strict > + * checks enabled -- no ifs or buts. > + */ > + if ((p->flags & flags) == flags) { > + excluded = true; > + break; > + } > + } > + read_unlock(&resource_lock); > + > + return excluded; > +} > + > #ifdef CONFIG_STRICT_DEVMEM > static int strict_iomem_checks = 1; > #else > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index 5ddd575159fb..d0ce6e23a6db 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -1780,7 +1780,9 @@ config STRICT_DEVMEM > access to this is obviously disastrous, but specific access can > be used by people debugging the kernel. Note that with PAT support > enabled, even in this case there are restrictions on /dev/mem > - use due to the cache aliasing requirements. > + use due to the cache aliasing requirements. Further, some drivers > + will still restrict access to some physical memory regions either > + already used or to be used in the future as system RAM. > > If this option is switched on, and IO_STRICT_DEVMEM=n, the /dev/mem > file only allows userspace access to PCI space and the BIOS code and > -- > 2.31.1 >
David Hildenbrand
2021-Aug-25 07:23 UTC
[PATCH v2 1/3] /dev/mem: disallow access to explicitly excluded system RAM regions
On 25.08.21 02:58, Dan Williams wrote:> On Mon, Aug 16, 2021 at 7:25 AM David Hildenbrand <david at redhat.com> wrote: >> >> virtio-mem dynamically exposes memory inside a device memory region as >> system RAM to Linux, coordinating with the hypervisor which parts are >> actually "plugged" and consequently usable/accessible. On the one hand, the >> virtio-mem driver adds/removes whole memory blocks, creating/removing busy >> IORESOURCE_SYSTEM_RAM resources, on the other hand, it logically (un)plugs >> memory inside added memory blocks, dynamically either exposing them to >> the buddy or hiding them from the buddy and marking them PG_offline. >> >> virtio-mem wants to make sure that in a sane environment, nobody >> "accidentially" accesses unplugged memory inside the device managed >> region. After /proc/kcore has been sanitized and /dev/kmem has been >> removed, /dev/mem is the remaining interface that still allows uncontrolled >> access to the device-managed region of virtio-mem devices from user >> space. >> >> There is no known sane use case for mapping virtio-mem device memory >> via /dev/mem while virtio-mem driver concurrently (un)plugs memory inside >> that region. So once the driver was loaded and detected the device >> along the device-managed region, we just want to disallow any access via >> /dev/mem to it. >> >> Let's add the basic infrastructure to exclude some physical memory >> regions completely from /dev/mem access, on any architecture and under >> any system configuration (independent of CONFIG_STRICT_DEVMEM and >> independent of "iomem="). > > I'm certainly on team "/dev/mem considered harmful", but this approach > feels awkward. It feels wrong for being non-committal about whether > CONFIG_STRICT_DEVMEM is in wide enough use that the safety can be > turned on all the time, and the configuration option dropped, or there > are users clinging onto /dev/mem where they expect to be able to build > a debug kernel to turn all of these restrictions off, even the > virtio-mem ones. This splits the difference and says some /dev/mem > accesses are always disallowed for "reasons", but I could say the same > thing about pmem, there's no sane reason to allow /dev/mem which has > no idea about the responsibilities of properly touching pmem to get > access to it.For virtio-mem, there is no use case *and* access could be harmful; I don't even want to allow if for debugging purposes. If you want to inspect virtio-mem device memory content, use /proc/kcore, which performs proper synchronized access checks. Modifying random virtio-mem memory via /dev/mem in a debug kernel will not be possible: if you really have to play with fire, use kdb or better don't load the virtio-mem driver during boot, such that the kernel won't even be making use of device memory. I don't want people disabling CONFIG_STRICT_DEVMEM, or booting with "iomem=relaxed", and "accidentally" accessing any of virtio-mem memory via /dev/mem, while it gets concurrently plugged/unplugged by the virtio-mem driver. Not even for debugging purposes. We disallow mapping to some other regions independent of CONFIG_STRICT_DEVMEM already, so the idea to ignore CONFIG_STRICT_DEVMEM is not completely new: "Note that with PAT support enabled, even in this case there are restrictions on /dev/mem use due to the cache aliasing requirements." Maybe you even want to do something similar with PMEM now that there is infrastructure for it and just avoid having to deal with revoking /dev/mem mappings later. I think there are weird debugging/educational setups [1] that still require CONFIG_STRICT_DEVMEM=n even with iomem=relaxed. Take a look at lib/devmem_is_allowed.c:devmem_is_allowed(), it disallows any access to (what's currently added as) System RAM. It might just do what people want when dealing with system RAM that doesn't suddenly vanish , so I don't ultimately see why we should remove CONFIG_STRICT_DEVMEM=n. [1] https://bakhi.github.io/devmem/ Thanks! -- Thanks, David / dhildenb