The patch titled Subject: mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped has been added to the -mm mm-unstable branch. Its filename is mm-fix-a-uaf-when-vma-mm-is-freed-after-vma-vm_refcnt-got-dropped.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Suren Baghdasaryan surenb@google.com Subject: mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped Date: Mon, 28 Jul 2025 10:53:55 -0700
By inducing delays in the right places, Jann Horn created a reproducer for a hard to hit UAF issue that became possible after VMAs were allowed to be recycled by adding SLAB_TYPESAFE_BY_RCU to their cache.
Race description is borrowed from Jann's discovery report: lock_vma_under_rcu() looks up a VMA locklessly with mas_walk() under rcu_read_lock(). At that point, the VMA may be concurrently freed, and it can be recycled by another process. vma_start_read() then increments the vma->vm_refcnt (if it is in an acceptable range), and if this succeeds, vma_start_read() can return a recycled VMA.
In this scenario where the VMA has been recycled, lock_vma_under_rcu() will then detect the mismatching ->vm_mm pointer and drop the VMA through vma_end_read(), which calls vma_refcount_put(). vma_refcount_put() drops the refcount and then calls rcuwait_wake_up() using a copy of vma->vm_mm. This is wrong: It implicitly assumes that the caller is keeping the VMA's mm alive, but in this scenario the caller has no relation to the VMA's mm, so the rcuwait_wake_up() can cause UAF.
The diagram depicting the race: T1 T2 T3 == == == lock_vma_under_rcu mas_walk <VMA gets removed from mm> mmap <the same VMA is reallocated> vma_start_read __refcount_inc_not_zero_limited_acquire munmap __vma_enter_locked refcount_add_not_zero vma_end_read vma_refcount_put __refcount_dec_and_test rcuwait_wait_event <finish operation> rcuwait_wake_up [UAF]
Note that rcuwait_wait_event() in T3 does not block because refcount was already dropped by T1. At this point T3 can exit and free the mm causing UAF in T1. To avoid this we move vma->vm_mm verification into vma_start_read() and grab vma->vm_mm to stabilize it before vma_refcount_put() operation.
Link: https://lkml.kernel.org/r/20250728175355.2282375-1-surenb@google.com Fixes: 3104138517fc ("mm: make vma cache SLAB_TYPESAFE_BY_RCU") Signed-off-by: Suren Baghdasaryan surenb@google.com Reported-by: Jann Horn jannh@google.com Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m... Cc: Jann Horn jannh@google.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Vlastimil Babka vbabka@suse.cz Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
include/linux/mmap_lock.h | 23 +++++++++++++++++++++++ mm/mmap_lock.c | 10 +++------- 2 files changed, 26 insertions(+), 7 deletions(-)
--- a/include/linux/mmap_lock.h~mm-fix-a-uaf-when-vma-mm-is-freed-after-vma-vm_refcnt-got-dropped +++ a/include/linux/mmap_lock.h @@ -12,6 +12,7 @@ extern int rcuwait_wake_up(struct rcuwai #include <linux/tracepoint-defs.h> #include <linux/types.h> #include <linux/cleanup.h> +#include <linux/sched/mm.h>
#define MMAP_LOCK_INITIALIZER(name) \ .mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock), @@ -183,6 +184,28 @@ static inline struct vm_area_struct *vma }
rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_); + + /* + * If vma got attached to another mm from under us, that mm is not + * stable and can be freed in the narrow window after vma->vm_refcnt + * is dropped and before rcuwait_wake_up(mm) is called. Grab it before + * releasing vma->vm_refcnt. + */ + if (unlikely(vma->vm_mm != mm)) { + /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */ + struct mm_struct *other_mm = vma->vm_mm; + /* + * __mmdrop() is a heavy operation and we don't need RCU + * protection here. Release RCU lock during these operations. + */ + rcu_read_unlock(); + mmgrab(other_mm); + vma_refcount_put(vma); + mmdrop(other_mm); + rcu_read_lock(); + return NULL; + } + /* * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result. * False unlocked result is impossible because we modify and check --- a/mm/mmap_lock.c~mm-fix-a-uaf-when-vma-mm-is-freed-after-vma-vm_refcnt-got-dropped +++ a/mm/mmap_lock.c @@ -164,8 +164,7 @@ retry: */
/* Check if the vma we locked is the right one. */ - if (unlikely(vma->vm_mm != mm || - address < vma->vm_start || address >= vma->vm_end)) + if (unlikely(address < vma->vm_start || address >= vma->vm_end)) goto inval_end_read;
rcu_read_unlock(); @@ -236,11 +235,8 @@ retry: goto fallback; }
- /* - * Verify the vma we locked belongs to the same address space and it's - * not behind of the last search position. - */ - if (unlikely(vma->vm_mm != mm || from_addr >= vma->vm_end)) + /* Verify the vma is not behind of the last search position. */ + if (unlikely(from_addr >= vma->vm_end)) goto fallback_unlock;
/* _
Patches currently in -mm which might be from surenb@google.com are
mm-fix-a-uaf-when-vma-mm-is-freed-after-vma-vm_refcnt-got-dropped.patch