[PATCH 1/2] drm/amdgpu: fix integer overflow in amdgpu_gem_align_pitch()

[Werner Kasselman]

amdgpu_gem_align_pitch() uses signed int for the pitch calculation.
When alignment rounding pushes the width to a boundary value (e.g.,
2^30 for cpp=4), the multiplication 'aligned * cpp' overflows signed
32-bit int, producing 0 or a negative value.

The overflow guard in drm_mode_create_dumb() validates width * cpp
BEFORE the driver callback, but amdgpu_mode_dumb_create() bypasses the
generic drm_mode_size_dumb() helper and performs its own alignment
rounding, which can push the pitch past the pre-validated range.

A zero pitch propagates to a zero-size GEM object allocation via
amdgpu_gem_object_create(). The 0-byte BO passes
amdgpu_bo_validate_size() (since 0 < man->size) and is returned to
userspace with a valid handle. This object can then be mmap'd or
referenced in GPU command submissions, potentially causing out-of-bounds
access to adjacent slab memory.

DRM_IOCTL_MODE_CREATE_DUMB requires no DRM authentication, so any local
user with access to /dev/dri/renderD* can trigger this with e.g.
width=1073741760, bpp=32, height=1.

Add an overflow check in amdgpu_gem_align_pitch() to detect when
'aligned * cpp' would exceed INT_MAX, returning 0 in that case. Add
corresponding checks in amdgpu_mode_dumb_create() to reject pitch=0
and size=0 with -EINVAL.

The proper long-term fix is to convert amdgpu to use
drm_mode_size_dumb() which centralizes pitch/size calculation with
proper overflow guards, as is being done for other drivers in Thomas
Zimmermann's dumb-buffer series.

Found via AST-based call-graph analysis using sqry.

Fixes: 087451f372bf ("drm/amdgpu: use generic fb helpers instead of setting up AMD own's.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Werner Kasselman <werner@xxxxxxxxxxx>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
index a6107109a2b8..b4341abba20c 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
@@ -1246,6 +1246,15 @@ static int amdgpu_gem_align_pitch(struct amdgpu_device *adev,
 
 	aligned += pitch_mask;
 	aligned &= ~pitch_mask;
+
+	/* Sanity check to avoid integer overflow in aligned * cpp.
+	 * The caller (drm_mode_create_dumb) validates width * cpp fits
+	 * in u32 before alignment, but rounding up can push aligned
+	 * past INT_MAX / cpp, causing signed overflow to 0 or negative.
+	 */
+	if (aligned > INT_MAX / (cpp ? cpp : 1) || aligned <= 0)
+		return 0;
+
 	return aligned * cpp;
 }
 
@@ -1273,8 +1282,12 @@ int amdgpu_mode_dumb_create(struct drm_file *file_priv,
 
 	args->pitch = amdgpu_gem_align_pitch(adev, args->width,
 					     DIV_ROUND_UP(args->bpp, 8), 0);
+	if (!args->pitch)
+		return -EINVAL;
 	args->size = (u64)args->pitch * args->height;
 	args->size = ALIGN(args->size, PAGE_SIZE);
+	if (!args->size)
+		return -EINVAL;
 	domain = amdgpu_bo_get_preferred_domain(adev,
 				amdgpu_display_supported_domains(adev, flags));
 	r = amdgpu_gem_object_create(adev, args->size, 0, domain, flags,
-- 
2.43.0