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Tue, 13 Oct 2026 10:07:52 -0700 (PDT) Received: from dev1.kernelcare.com ([2a01:4f8:172:1c85::2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a18bf1358asm379560935e9.4.2026.10.13.10.07.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 13 Oct 2026 10:07:51 -0700 (PDT) From: Andrey Misiurov To: Juergen Gross , Stefano Stabellini Cc: Oleksandr Tyshchenko , Wentao Liang , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Andrey Misiurov , stable@vger.kernel.org Subject: [PATCH] xen/gntdev: fix deadlock between map ioctl and mmap Date: Tue, 13 Oct 2026 19:07:45 +0200 Message-ID: <20261013170745.583279-1-andrey.misyrov@gmail.com> X-Mailer: git-send-email 2.43.7 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Commit 45ca1afe2fd1 ("xen/gntdev: fix error handling in ioctl") moved copy_to_user() in gntdev_ioctl_map_grant_ref() under priv->lock. A fault in copy_to_user() takes mmap_lock, while gntdev_mmap() is called with mmap_lock held and takes priv->lock: ioctl thread mmap thread mutex_lock(&priv->lock) mmap_write_lock(mm) gntdev_mmap() mutex_lock(&priv->lock) copy_to_user() faults mmap_read_lock(mm) Reproduced on Debian 6.12.111 in a Xen guest, with the ioctl argument on a MAP_PRIVATE file page that has not been written yet. sysrq-w: task:repro state:D stack:0 pid:4479 tgid:4477 ppid:1 flags:0x00000000 rwsem_down_read_slowpath+0x25c/0x480 down_read_killable+0x48/0xc0 lock_mm_and_find_vma+0x96/0x250 do_user_addr_fault+0x12d/0x620 exc_page_fault+0x7e/0x180 asm_exc_page_fault+0x26/0x30 _copy_to_user+0x36/0x50 gntdev_ioctl+0x3b9/0xa00 [xen_gntdev] __x64_sys_ioctl+0x97/0xc0 task:repro state:D stack:0 pid:4480 tgid:4477 ppid:1 flags:0x00000000 __mutex_lock.constprop.0+0x41b/0x720 gntdev_mmap+0x4d/0x330 [xen_gntdev] __mmap_region+0x697/0xb10 mmap_region+0x78/0xa0 do_mmap+0x45d/0x630 vm_mmap_pgoff+0xf8/0x1c0 Drop priv->lock before copy_to_user() again. On failure, re-take it and remove the map by index, as IOCTL_GNTDEV_UNMAP_GRANT_REF does, since another thread may have unmapped it meanwhile. Fixes: 45ca1afe2fd1 ("xen/gntdev: fix error handling in ioctl") Cc: stable@vger.kernel.org Signed-off-by: Andrey Misiurov --- drivers/xen/gntdev.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c index b71f5ba..1ad0220 100644 --- a/drivers/xen/gntdev.c +++ b/drivers/xen/gntdev.c @@ -670,15 +670,19 @@ static long gntdev_ioctl_map_grant_ref(struct gntdev_priv *priv, mutex_lock(&priv->lock); gntdev_add_map(priv, map); op.index = map->index << PAGE_SHIFT; + mutex_unlock(&priv->lock); if (copy_to_user(u, &op, sizeof(op)) != 0) { - list_del(&map->next); + mutex_lock(&priv->lock); + map = gntdev_find_map_index(priv, op.index >> PAGE_SHIFT, + op.count); + if (map) + list_del(&map->next); mutex_unlock(&priv->lock); gntdev_put_map(priv, map); return -EFAULT; } - mutex_unlock(&priv->lock); return 0; } base-commit: 61b6d1df1d6234c38a6ea7e4f58e7da035ef6ab4 -- 2.43.7