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([156.223.77.192]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4804dbc0d84sm52711215e9.6.2026.01.24.08.46.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 Jan 2026 08:46:38 -0800 (PST) From: Osama Abdelkader To: Simona Vetter , Helge Deller , Thomas Zimmermann , Lee Jones , "Daniel Thompson (RISCstar)" , Murad Masimov , Quanmin Yan , Yongzhen Zhang , Osama Abdelkader , linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Cc: syzbot+55e03490a0175b8dd81d@syzkaller.appspotmail.com Subject: [PATCH v2] fbdev: avoid out-of-bounds read in fb_pad_unaligned_buffer() Date: Sat, 24 Jan 2026 17:46:30 +0100 Message-ID: <20260124164633.20444-1-osama.abdelkader@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-fbdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit fb_pad_unaligned_buffer() unconditionally reads and advances the source pointer for the final byte of each row, even when no bits from that byte are actually consumed. When shift_high >= mod, the remaining bits do not cross a byte boundary, but the code still accesses the next source byte. This can lead to out-of-bounds reads under malformed geometry, as reported by syzbot. Fix this by only accessing and consuming the final source byte when it contributes bits (shift_high < mod). This fixes the KASAN slab-out-of-bounds read reported by syzkaller: https://syzkaller.appspot.com/bug?extid=55e03490a0175b8dd81d Reported-by: syzbot+55e03490a0175b8dd81d@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=55e03490a0175b8dd81d Signed-off-by: Osama Abdelkader --- v2: address the real issue (shift_high >= mod) condition. --- drivers/video/fbdev/core/fbmem.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c index eff757ebbed1..d125c3db37a1 100644 --- a/drivers/video/fbdev/core/fbmem.c +++ b/drivers/video/fbdev/core/fbmem.c @@ -100,7 +100,7 @@ EXPORT_SYMBOL(fb_pad_aligned_buffer); void fb_pad_unaligned_buffer(u8 *dst, u32 d_pitch, u8 *src, u32 idx, u32 height, u32 shift_high, u32 shift_low, u32 mod) { - u8 mask = (u8) (0xfff << shift_high), tmp; + u8 mask = (u8) (0xff << shift_high), tmp; int i, j; for (i = height; i--; ) { @@ -113,15 +113,18 @@ void fb_pad_unaligned_buffer(u8 *dst, u32 d_pitch, u8 *src, u32 idx, u32 height, dst[j+1] = tmp; src++; } - tmp = dst[idx]; - tmp &= mask; - tmp |= *src >> shift_low; - dst[idx] = tmp; + + /* Only consume another source byte if it contributes bits */ if (shift_high < mod) { + tmp = dst[idx]; + tmp &= mask; + tmp |= *src >> shift_low; + dst[idx] = tmp; tmp = *src << shift_high; dst[idx+1] = tmp; + src++; } - src++; + dst += d_pitch; } } -- 2.43.0