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fb_display[i]->mode to NULL when the mode is released To: Quanmin Yan , tzimmermann@suse.de Cc: simona@ffwll.ch, linux-kernel@vger.kernel.org, linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, wangkefeng.wang@huawei.com, zuoze1@huawei.com, sunnanyong@huawei.com References: <20251010081659.3609082-1-yanquanmin1@huawei.com> Content-Language: en-US From: Helge Deller Autocrypt: addr=deller@gmx.de; keydata= xsFNBF3Ia3MBEAD3nmWzMgQByYAWnb9cNqspnkb2GLVKzhoH2QD4eRpyDLA/3smlClbeKkWT HLnjgkbPFDmcmCz5V0Wv1mKYRClAHPCIBIJgyICqqUZo2qGmKstUx3pFAiztlXBANpRECgwJ r+8w6mkccOM9GhoPU0vMaD/UVJcJQzvrxVHO8EHS36aUkjKd6cOpdVbCt3qx8cEhCmaFEO6u CL+k5AZQoABbFQEBocZE1/lSYzaHkcHrjn4cQjc3CffXnUVYwlo8EYOtAHgMDC39s9a7S90L 69l6G73lYBD/Br5lnDPlG6dKfGFZZpQ1h8/x+Qz366Ojfq9MuuRJg7ZQpe6foiOtqwKym/zV dVvSdOOc5sHSpfwu5+BVAAyBd6hw4NddlAQUjHSRs3zJ9OfrEx2d3mIfXZ7+pMhZ7qX0Axlq Lq+B5cfLpzkPAgKn11tfXFxP+hcPHIts0bnDz4EEp+HraW+oRCH2m57Y9zhcJTOJaLw4YpTY 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9g866HA33zDRbwuLgnkCgvuKY+4DrP2TyGbQSGjVj4oABG0dOEzApjs+NIVYELWBneW5tzX/o IWUmK0tv+4iO7PFYetOKfK8U4e+K8Xgmbz7Cy9gcev3UtdRNsb8yPF0bTQ0bFxMwFDnKskStt Z+/KrW6iK+QJmtSxHhZ7EdPq6H/knKj1TVspptln6R8qTGgMdojR1s06kp+GMiYKsGhavvMr6 Lv4B4JoSriA9mDB8xS/bKFKLJrWBH0Kq2wVRDGjCEek+f+9szmHZ5S4zJ/PvSelyx0xuUdfZk w40T5NLJbsFoKEmM/yfwH/Un+ZFg97ugtNNB07evXPWUXmL0hcwPv/rwVPbW5yk8ugjIQlxiJ s0N17aLK1IaRVnzDXCY4hpiBzv3uBfm1GsPwcyDsPQGA876ZEpY8fCJtxPhX5IgSdVRP4wviI F3vTSOTOLLygrci3igZA9l0gvF3FOhq57m0IrRt8vsHg5q8JHu71sn9mGFoNYOyjsx09Mmr3u cCQyNd5eL5hJVlk1Y/1IAZcIuLaFeg10z4LDR2UsSwcIGPl5Yq6phtUQR/twNukPoCvPKA273 1DOoeTfzT5HtGmKAdbVctRJwKYnpwQ7ageexXutZGaBI8hd On 10/10/25 10:16, Quanmin Yan wrote: > Recently, we discovered the following issue through syzkaller: >=20 > BUG: KASAN: slab-use-after-free in fb_mode_is_equal+0x285/0x2f0 > Read of size 4 at addr ff11000001b3c69c by task syz.xxx > ... > Call Trace: > > dump_stack_lvl+0xab/0xe0 > print_address_description.constprop.0+0x2c/0x390 > print_report+0xb9/0x280 > kasan_report+0xb8/0xf0 > fb_mode_is_equal+0x285/0x2f0 > fbcon_mode_deleted+0x129/0x180 > fb_set_var+0xe7f/0x11d0 > do_fb_ioctl+0x6a0/0x750 > fb_ioctl+0xe0/0x140 > __x64_sys_ioctl+0x193/0x210 > do_syscall_64+0x5f/0x9c0 > entry_SYSCALL_64_after_hwframe+0x76/0x7e >=20 > Based on experimentation and analysis, during framebuffer unregistration= , > only the memory of fb_info->modelist is freed, without setting the > corresponding fb_display[i]->mode to NULL for the freed modes. This lead= s > to UAF issues during subsequent accesses. Here's an example of reproduct= ion > steps: > 1. With /dev/fb0 already registered in the system, load a kernel module > to register a new device /dev/fb1; > 2. Set fb1's mode to the global fb_display[] array (via FBIOPUT_CON2FBMA= P); > 3. Switch console from fb to VGA (to allow normal rmmod of the ko); > 4. Unload the kernel module, at this point fb1's modelist is freed, leav= ing > a wild pointer in fb_display[]; > 5. Trigger the bug via system calls through fb0 attempting to delete a m= ode > from fb0. >=20 > Add a check in do_unregister_framebuffer(): if the mode to be freed exis= ts > in fb_display[], set the corresponding mode pointer to NULL. >=20 > Signed-off-by: Quanmin Yan > ---=20 > drivers/video/fbdev/core/fbcon.c | 19 +++++++++++++++++++ > drivers/video/fbdev/core/fbmem.c | 1 + > include/linux/fbcon.h | 2 ++ > 3 files changed, 22 insertions(+) applied. Thanks! Helge