Hi Rob, On Tue, Jan 03, 2023 at 03:49:46PM -0800, Rob Clark wrote: > From: Rob Clark <robdclark@xxxxxxxxxxxx> > > gem_context_register() makes the context visible to userspace, and which > point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl. > So we need to ensure that nothing uses the ctx ptr after this. And we > need to ensure that adding the ctx to the xarray is the *last* thing > that gem_context_register() does with the ctx pointer. > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx> Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx> I also agree with Tvrtko that we should add Stable: and Fixes:. One little thing, "user after free" is clearer that UAF :) Thanks, Andi > --- > drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------ > 1 file changed, 18 insertions(+), 6 deletions(-) > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c > index 7f2831efc798..6250de9b9196 100644 > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c > @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915) > init_contexts(&i915->gem.contexts); > } > > +/* > + * Note that this implicitly consumes the ctx reference, by placing > + * the ctx in the context_xa. > + */ > static void gem_context_register(struct i915_gem_context *ctx, > struct drm_i915_file_private *fpriv, > u32 id) > @@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx, > snprintf(ctx->name, sizeof(ctx->name), "%s[%d]", > current->comm, pid_nr(ctx->pid)); > > - /* And finally expose ourselves to userspace via the idr */ > - old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL); > - WARN_ON(old); > - > spin_lock(&ctx->client->ctx_lock); > list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list); > spin_unlock(&ctx->client->ctx_lock); > @@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx, > spin_lock(&i915->gem.contexts.lock); > list_add_tail(&ctx->link, &i915->gem.contexts.list); > spin_unlock(&i915->gem.contexts.lock); > + > + /* And finally expose ourselves to userspace via the idr */ > + old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL); > + WARN_ON(old); > } > > int i915_gem_context_open(struct drm_i915_private *i915, > @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv, > if (IS_ERR(ctx)) > return ctx; > > + /* > + * One for the xarray and one for the caller. We need to grab > + * the reference *prior* to making the ctx visble to userspace > + * in gem_context_register(), as at any point after that > + * userspace can try to race us with another thread destroying > + * the context under our feet. > + */ > + i915_gem_context_get(ctx); > + > gem_context_register(ctx, file_priv, id); > > old = xa_erase(&file_priv->proto_context_xa, id); > GEM_BUG_ON(old != pc); > proto_context_close(file_priv->dev_priv, pc); > > - /* One for the xarray and one for the caller */ > - return i915_gem_context_get(ctx); > + return ctx; > } > > struct i915_gem_context * > -- > 2.38.1
