From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Subject: idr: fix invalid ptr dereference on item delete If the radix tree underlying the IDR happens to be full and we attempt to remove an id which is larger than any id in the IDR, we will call __radix_tree_delete() with an uninitialised 'slot' pointer, at which point anything could happen. This was easiest to hit with a single entry at id 0 and attempting to remove a non-0 id, but it could have happened with 64 entries and attempting to remove an id >= 64. Roman said: The syzcaller test boils down to opening /dev/kvm, creating an eventfd, and calling a couple of KVM ioctls. None of this requires superuser. And the result is dereferencing an uninitialized pointer which is likely a crash. The specific path caught by syzbot is via KVM_HYPERV_EVENTD ioctl which is new in 4.17. But I guess there are other user-triggerable paths, so cc:stable is probably justified. Matthew added: We have around 250 calls to idr_remove() in the kernel today. Many of them pass an ID which is embedded in the object they're removing, so they're safe. Picking a few likely candidates: drivers/firewire/core-cdev.c looks unsafe; the ID comes from an ioctl. drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c is similar drivers/atm/nicstar.c could be taken down by a handcrafted packet Link: http://lkml.kernel.org/r/20180518175025.GD6361@xxxxxxxxxxxxxxxxxxxxxx Fixes: 0a835c4f090a ("Reimplement IDR and IDA using the radix tree") Reported-by: <syzbot+35666cba7f0a337e2e79@xxxxxxxxxxxxxxxxxxxxxxxxx> Debugged-by: Roman Kagan <rkagan@xxxxxxxxxxxxx> Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- lib/radix-tree.c | 4 +++- tools/testing/radix-tree/idr-test.c | 7 +++++++ 2 files changed, 10 insertions(+), 1 deletion(-) diff -puN lib/radix-tree.c~idr-fix-invalid-ptr-dereference-on-item-delete lib/radix-tree.c --- a/lib/radix-tree.c~idr-fix-invalid-ptr-dereference-on-item-delete +++ a/lib/radix-tree.c @@ -2034,10 +2034,12 @@ void *radix_tree_delete_item(struct radi unsigned long index, void *item) { struct radix_tree_node *node = NULL; - void __rcu **slot; + void __rcu **slot = NULL; void *entry; entry = __radix_tree_lookup(root, index, &node, &slot); + if (!slot) + return NULL; if (!entry && (!is_idr(root) || node_tag_get(root, node, IDR_FREE, get_slot_offset(node, slot)))) return NULL; diff -puN tools/testing/radix-tree/idr-test.c~idr-fix-invalid-ptr-dereference-on-item-delete tools/testing/radix-tree/idr-test.c --- a/tools/testing/radix-tree/idr-test.c~idr-fix-invalid-ptr-dereference-on-item-delete +++ a/tools/testing/radix-tree/idr-test.c @@ -252,6 +252,13 @@ void idr_checks(void) idr_remove(&idr, 3); idr_remove(&idr, 0); + assert(idr_alloc(&idr, DUMMY_PTR, 0, 0, GFP_KERNEL) == 0); + idr_remove(&idr, 1); + for (i = 1; i < RADIX_TREE_MAP_SIZE; i++) + assert(idr_alloc(&idr, DUMMY_PTR, 0, 0, GFP_KERNEL) == i); + idr_remove(&idr, 1 << 30); + idr_destroy(&idr); + for (i = INT_MAX - 3UL; i < INT_MAX + 1UL; i++) { struct item *item = item_create(i, 0); assert(idr_alloc(&idr, item, i, i + 10, GFP_KERNEL) == i); _
