From 73714beab12fec056f3b38a7c2bc35a520405953 Mon Sep 17 00:00:00 2001 From: Gerd Hoffmann Date: Fri, 29 Apr 2016 07:02:47 +0200 Subject: [PATCH 2/6] vga: fix banked access bounds checking (CVE-2016-3710) RH-Author: Gerd Hoffmann Message-id: <1461913371-3145-3-git-send-email-kraxel@redhat.com> Patchwork-id: 70302 O-Subject: [virt-devel] [RHEL-7.2.z qemu-kvm PATCH 2/6] vga: fix banked access bounds checking (CVE-2016-3710) Bugzilla: 1331412 RH-Acked-by: Miroslav Rezanina RH-Acked-by: Laszlo Ersek RH-Acked-by: Dr. David Alan Gilbert vga allows banked access to video memory using the window at 0xa00000 and it supports a different access modes with different address calculations. The VBE bochs extentions support banked access too, using the VBE_DISPI_INDEX_BANK register. The code tries to take the different address calculations into account and applies different limits to VBE_DISPI_INDEX_BANK depending on the current access mode. Which is probably effective in stopping misprogramming by accident. But from a security point of view completely useless as an attacker can easily change access modes after setting the bank register. Drop the bogus check, add range checks to vga_mem_{readb,writeb} instead. Fixes: CVE-2016-3710 Reported-by: Qinghao Tang Signed-off-by: Gerd Hoffmann Signed-off-by: Miroslav Rezanina --- hw/display/vga.c | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-) diff --git a/hw/display/vga.c b/hw/display/vga.c index 48dad03..ba171ba 100644 --- a/hw/display/vga.c +++ b/hw/display/vga.c @@ -744,11 +744,7 @@ void vbe_ioport_write_data(void *opaque, uint32_t addr, uint32_t val) vbe_fixup_regs(s); break; case VBE_DISPI_INDEX_BANK: - if (s->vbe_regs[VBE_DISPI_INDEX_BPP] == 4) { - val &= (s->vbe_bank_mask >> 2); - } else { - val &= s->vbe_bank_mask; - } + val &= s->vbe_bank_mask; s->vbe_regs[s->vbe_index] = val; s->bank_offset = (val << 16); vga_update_memory_access(s); @@ -847,13 +843,21 @@ uint32_t vga_mem_readb(VGACommonState *s, hwaddr addr) if (s->sr[VGA_SEQ_MEMORY_MODE] & VGA_SR04_CHN_4M) { /* chain 4 mode : simplest access */ + assert(addr < s->vram_size); ret = s->vram_ptr[addr]; } else if (s->gr[VGA_GFX_MODE] & 0x10) { /* odd/even mode (aka text mode mapping) */ plane = (s->gr[VGA_GFX_PLANE_READ] & 2) | (addr & 1); - ret = s->vram_ptr[((addr & ~1) << 1) | plane]; + addr = ((addr & ~1) << 1) | plane; + if (addr >= s->vram_size) { + return 0xff; + } + ret = s->vram_ptr[addr]; } else { /* standard VGA latched access */ + if (addr * sizeof(uint32_t) >= s->vram_size) { + return 0xff; + } s->latch = ((uint32_t *)s->vram_ptr)[addr]; if (!(s->gr[VGA_GFX_MODE] & 0x08)) { @@ -910,6 +914,7 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val) plane = addr & 3; mask = (1 << plane); if (s->sr[VGA_SEQ_PLANE_WRITE] & mask) { + assert(addr < s->vram_size); s->vram_ptr[addr] = val; #ifdef DEBUG_VGA_MEM printf("vga: chain4: [0x" TARGET_FMT_plx "]\n", addr); @@ -923,6 +928,9 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val) mask = (1 << plane); if (s->sr[VGA_SEQ_PLANE_WRITE] & mask) { addr = ((addr & ~1) << 1) | plane; + if (addr >= s->vram_size) { + return; + } s->vram_ptr[addr] = val; #ifdef DEBUG_VGA_MEM printf("vga: odd/even: [0x" TARGET_FMT_plx "]\n", addr); @@ -996,6 +1004,9 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val) mask = s->sr[VGA_SEQ_PLANE_WRITE]; s->plane_updated |= mask; /* only used to detect font change */ write_mask = mask16[mask]; + if (addr * sizeof(uint32_t) >= s->vram_size) { + return; + } ((uint32_t *)s->vram_ptr)[addr] = (((uint32_t *)s->vram_ptr)[addr] & ~write_mask) | (val & write_mask); -- 1.8.3.1