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bit_putcs to fix vmalloc-out-of-bounds To: Albin Babu Varghese Cc: Simona Vetter , syzbot+48b0652a95834717f190@syzkaller.appspotmail.com, linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org References: <20250927075010.119671-1-albinbabuvarghese20@gmail.com> <5ab00319-e43e-4000-8814-c7d67f384c53@gmx.de> 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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Ok. =20 > I added debug prints to check what's happening and the clamping in > set_selection() is working and the values coming through are within boun= ds. But > the crash still happens when you remap the framebuffer because of a slig= ht > overflow. >=20 > I also discovered that when image.width is clipped on the X-axis, the ch= aracter > count (cnt) must also be updated to match, otherwise bit_putcs_aligned() > receives mismatched buffer size and character count parameters, causing > out-of-bounds writes. >=20 > So I changed it to something like this: >=20 > + if (image.dx >=3D info->var.xres) > + break; > + if (image.dx + image.width > info->var.xres) { > + image.width =3D info->var.xres - image.dx; > + cnt =3D image.width / vc->vc_font.width; > + if (cnt =3D=3D 0) > + break; > + image.width =3D cnt * vc->vc_font.width; > + } Looks good. > I tested it in syzbot, with the syzkaller reproducer, and also manually = in QEMU > and verified that the buffer switches from tty1 to tty2 work correctly. >=20 > I couldn=E2=80=99t find a dedicated fbdev/fbcon test suite. Beyond kself= tests, do you > recommend anything specific before sending v2? There is Geert's fbdev test tool (https://git.kernel.org/pub/scm/linux/ker= nel/git/geert/fbtest.git/), but it does not involve testing of fbcon. Helge